Canon EOS-1D X Manual (en)

Registering Background Music to a Camera Memory Card
You can register WAV-format music files, saved to your computer, to the 
camera’s memory card as background music. Registered background 
music can be played together with a Video Snapshot Album or 
slideshow played on the camera.
Connect your camera and computer. Start up EU and 
click [Camera settings].
The [Camera settings] window appears.
Click [Register Background Music].
The [Register Background Music] window appears.
When the [EOS Sample Music] folder is installed to your 
computer, five tracks appear in [Background music to register] 
the first time as EOS Sample Music.
When the camera is in movie shooting mode, registering 
background music is not possible.
Edit [Background music to register].
Adding New Music Files
Click the [Add] button, select a music file in the [Open] dialog box 
that appears, and then click the [Open] button.
A music file is added to [Background music to register].
You can also directly drag and drop a music file saved anywhere 
in your computer to add it to [Background music to register].
Music can be added only one track at a time.
A total of 20 tracks can be added, each up to 29 minutes 59 
seconds long.
Register Background Music window