This is an option that you can enable or disable for the background music in the menus. You can adjust the loudness with the audio bar.

It might be useful to disable front-end music if you use “random video” screensaver, otherwise you will get both sound outputs (screensaver and front-end) at the same time.

Starting with 5.23, you can put sub-directories in /userdata/music/ in order to have system-specific musics only played while you are browsing a system. For example, put in /userdata/music/nes/ a set of MP3s you want to listen to, only when you are browsing your NES games catalog. And MP3s in /userdata/music/mame/ folder will be played only when you browse the MAME games.

Once you have the MP3s ordered by system subfolders, you can still play them all regardless of what system you are browsing by turning “off” the option in SOUND SETTINGS → ONLY PLAY SYSTEM-SPECIFIC MUSIC FOLDER.