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

1) Confirm the audio output destination setting
Open [Audio] in the rekordbox Preferences and check that the [audio] and [output channel] are selected correctly.
For details, refer to [Checking audio settings] in [Starting the system] in the Quick Start Guide.
*If you can’t select the correct choice because it isn’t displayed or an error has occurred, etc., refer to [Troubleshooting > "I can't set [AUDIO] in the rekordbox Preferences correctly."].
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 MIXING 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 [Connections] in [Operation] 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 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 rekordbox and disconnect the unit from your computer.
2. Open the control panel (or [Start] > [Settings] > [Apps]) and uninstall [DDJ-800 Driver].
3. Close Windows.
4. Disconnect other USB devices, USB hubs, etc.
5. Start Windows.
6. Download the 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 unit to another port without using a USB hub, etc.

If you still can't set [AUDIO], the problem may be caused by operating system settings, etc.
Refer to case 3 and 6 in the guide [here].

■Mac
1. Close rekordbox and disconnect the unit from your computer.
2. Download the driver software from [here].
3. Double-click [DDJ-800 Uninstaller] to uninstall it.
4. Close your Mac and disconnect other USB devices, USB hubs, etc.
5. Start your Mac.
6. Double-click [DDJ-800 AudioDriver.pkg] to reinstall it.
7. Connect the unit to another port without using a USB hub, etc.
8. Follow the instructions in the troubleshooting guide [here].
* Unnecessary for macOS Sierra 10.12

rekordbox 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 5 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 MASTER TEMPO.

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 "Master Attenuator Level setting" in "Changing the settings" in the Operating Instructions.
If sound skipping, sound cutting, or noise occurs, see [Troubleshooting > "Sound skips, cuts, or is affected by noise."]

You can read the Operating Instructions [here].

This may occur when a line level output device (e.g. a multi player) is connected to the unit while the LINE/PHONO switch is set to [PHONO].
In this case, switch to [LINE].

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 rekordbox [Preferences] < [Audio] < [Buffer size].
(Windows users can adjust it in [Preferences] < [Audio] < [Setting Utility] < [ASIO tab].)
First, select a large value such as 1024. 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 unit 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 rekordbox, it may cause the issue.
Read details on system requirements [here].

If you can't play audio files with rekordbox, check if the following applies:
- The file format is not supported by rekordbox.
- 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, see "Management" and "Music files that can be loaded and played (file formats)" in the rekordbox Operating Instructions.
You can read the rekordbox Operating Instructions[here].

Three main factors can cause this and there are three possible countermeasures.
■When the Windows built-in sound device setting is the cause
Refer to case 4 in the troubleshooting guide [here].

■When the connection or other equipment is the cause
An unstable connection with your PC/Mac may cause this problem.
To solve this, shut down your PC/Mac and disconnect all other USB devices and USB hubs.
Then, connect the unit 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 rekordbox, it may cause the issue.
Read details on system requirements [here].

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.

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.
To turn it on/off, open rekordbox [Preferences] > [Controller] > [Mixer].

For details, refer to the Operating Instructions > "Changing the settings" > "Changing the settings with rekordbox".
You can read the Operating Instructions [here].