Menu Sound effects
The N64FlashcartMenu has the ability to play sound effects during navigation. It is not currently possible to customize them. It is disabled by default but can be enabled from the N64FlashcartMenu settings menu.
Menu Background music
The N64FlashcartMenu does not currently support background music. It would not be possible to support custom music, but in future may add a simple background sound similar to that used by the GC or Wii.
Tip
Check the Pull Requests section for work towards it.
Playing music
MP3 playback is supported, see here