There are three things to consider: settings, operation and connection.
Please check each in the following order:

1) Confirm the settings on the Serato DJ Pro screen
1.　Check if [Error: Failed to connect audio!!] or [Driver Not Installed] message shows at the bottom of the screen.
If it is shown, read the DDJ-REV7 FAQ > [Troubleshooting > "The message [Error: Failed to connect audio!!] is shown, or the virtual deck disappears immediately on the Serato DJ Pro screen."]
2. Click the icon in [My Serato] on the upper right of the screen.
3. Check that the [Serato DJ Pro is activated] and [DDJ-REV7] are shown on the screen.
If not, read the DDJ-REV7 FAQ >[Troubleshooting > "The Serato DJ Pro screen does not react anything when the DDJ-REV7 is connected."]
2) Check the volume level operation
1. Play a track.
2. Set the TRIM knob to the center (12 o’clock).
3. Slide the channel fader to the top.
4. Slide the crossfader to the center.
5. Turn the MASTER LEVEL knob to 2 o'clock.
If the MASTER LEVEL indicator lights up more than half way, there’s no problem in the settings and operation.
If you don’t hear any sound from the headphones at this point, check the following:
6. Turn the HEADPHONES MIX knob to the center.
7. Turn the HEADPHONES LEVEL knob all the way to the left.
8. Press the MASTER CUE button.
9. Turn the HEADPHONES LEVEL knob slowly to the right.

3) Check the connection of speakers and headphones
For details, refer to [Basic operation] and [Example of connections] in in the Quick Start Guide.
Even if you’ve already connected them, disconnect and reconnect them to make sure they’re properly connected.
*You can't output audio from PC/Mac speakers.

You can read the Quick Start Guide [here].

The installation of the driver software may be incomplete or the connection may be causing a problem.
If you've already installed the driver software, try again with the following procedure:

■ Windows
1. Close Serato DJ Pro and disconnect the DDJ-REV7 from your computer.
2. Open the control panel (or [Start] > [Settings] > [Apps]) and uninstall [DDJ-REV7 Driver].
3. Close the Windows
4. Disconnect other USB devices, USB hubs, etc.
5. Start the Windows
6. Download driver software from [here]
7. Install the driver software again.
　*An error will occur when the license agreement screen is displayed and you try to proceed without clicking "I agree".
8. Connect the DDJ-REV7 to another port without using a USB hub, etc.

If you still have the problem, it may be caused by operating system sound settings, etc.
Remove all other USB devices from your computer.
Follow the instructions on the troubleshooting guide Case 3-1 "Check the default device selected in the [Sound] panel" or Case 6 [here]
*The guide is for rekordbox users, however you can fix the problem in the same manner.

■ Mac
1. Close Serato DJ Pro and disconnect the DDJ-REV7 from your computer.
2. Download the driver software from [here].
3. Double-click [DDJ-REV7 Uninstaller] to uninstall it.
4. Close your Mac and disconnect other USB devices, USB hubs, etc.
5. Start your Mac.
6. Double-click [DDJ-REV7 AudioDriver.pkg] to reinstall it.
7. Connect the DDJ-REV7 to another port without using a USB hub, etc.
8. Follow the instructions in the troubleshooting guide [here].
* Unnecessary for macOS Sierra 10.12 or earlier

If you still have the problem, it may be caused by operating system sound settings, etc.
Remove all other USB devices from the Mac, uninstall the virtual audio device etc, and restart the Mac.

Serato DJ Pro or your operating system may not recognize the unit as a controller.

Mac
Follow the instructions in the troubleshooting guide [here].

Windows
Refer to Case 3 in the guide [here].

You may solve the issue by doing the following:
- Disconnect any USB hubs and connect the unit directly to your PC/Mac.
- Disconnect any other USB devices.
- Quit all other applications.
- Connect to a different USB port.

If the volume level is too high, the sound may be affected by distortion or clipping.
Play a track and check the following:

1) Check the channel level indicator
Adjust the level with the TRIM knob so the orange color lights up in the louder parts of the track.
2) Check the MASTER LEVEL indicator
With the channel fader at the top, adjust the level with the MASTER LEVEL knob so the orange color lights up in the louder parts of the track.
3) Turn off KEY LOCK.

If distortion and sound clipping doesn’t improve, the output level may exceed the allowable input level of the connected speaker or amplifier.
In this case, you need to lower the output level with the MASTER attenuator.
For details, refer to the Instruction Manual > [Settings] > [Utility settings].
If sound skipping, sound cutting, or noise occurs, see the DDJ-REV7 FAQ > [Troubleshooting > "Sound skips, cuts, or is affected by noise.]

You can read the Instruction Manual [here].

The LINE/PHONO selector switch(es) may be incorrectly set.
To connect the following types of devices to the CH 1 or CH 2 terminals on the rear panel of the unit, select [LINE] with the relevant LINE/PHONO selector switch:

• CDJ
• Audio interface
• Turntable with line output (output level equivalent to that of a CDJ)

Three main factors can cause this and there are three possible countermeasures.

■When buffer size is the cause
If the buffer size is too low, sound cutting or noise may occur.
You can adjust buffer size in Serato DJ Pro [Setup] < [Audio] < [USB Buffer size].
First, select a large value such as 20 ms. If the problem clears up, adjust to a value small enough that you can’t feel a delay (latency).
*The appropriate value depends on the system environment.

■When the connection or other equipment is the cause
An unstable connection with your PC/Mac may cause these problems.
To solve this, shut down your PC/Mac and disconnect all other USB devices and USB hubs.
Then, connect the DDJ-REV7 directly to a different USB port.

■When your system (PC/Mac) is the cause
The following may solve the issue:
- Quit all other applications
- Disconnect from the internet and exit security software
- Turn off wireless connections such as Wi-Fi or Bluetooth
- Turn off power-saving settings

If your computer's system doesn’t meet the requirements of Serato DJ Pro, it may cause the issue.
Read details on system requirements [serato.com].

If you can't play audio files with Serato DJ Pro, check if the following applies:
- The file format is not supported by Serato DJ Pro.
- Only limited applications can play the audio file: e.g. the audio file was downloaded from Apple Music etc.
- The location of the audio file has changed.
- The music file is damaged.

For details, refer to Serato DJ Pro Manual.
You can read the Serato DJ Pro Manual [serato.com].

Talkover might be turned on.

When it’s on, this feature automatically reduces the volume when the microphone picks up the sound from the speakers. Try to use the microphone as far away from the speakers as possible.
This issue may be caused by static electricity etc. If you don't need to use Talkover, turn it off.

For details, refer to the Instruction Manual > [MIC].
You can read the Instruction Manual [here].

The virtual decks are shown after Serato DJ Pro recognizes the connected device.
When multiple ASIO drivers are installed or multiple audio devices are connected to your PC/Mac, it may take time for Serato DJ Pro to recognize the controller.
Please uninstall unnecessary ASIO drivers and virtual audio devices. Visit the website of relevant ASIO driver manufacturers for details of how to uninstall them. Remove audio devices other than DDJ-REV7.

If this happens immediately after you’ve updated the firmware, the update may have failed. Try updating the firmware again or disconnect the USB cable and AC adapter, wait for at least 1 minute and turn on the power. If this doesn’t solve the issue, request a repair.

The values may differ slightly due to different methods used for measuring the BPM. This is not a malfunction.

The following effects aren't applied to the headphones cue sound. Turn the [MIX] knob to the [MASTER] side to hear how the effect sounds on the master sound:

■ BEAT FX
Echo, Time Sweep Echo, Ducking Echo, Echo Out, Spiral, Helix Out, Delay, Reverb, Duck Down

■Filter FX
Dub Echo