To disable the annoying background music in MultiMAN, got to Settings, find Theme Audio, press controller button and choose DISABLED from the menu.
Alternatively, you delete the SOUND.BIN file from MultiMAN directory.
/dev_hdd0/game/BLES80608/USRDIR/
SOUND.BIN is a renamed mp3 file, so you if you want to change the music, you may just rename your tune of choice into SOUND.BIN and place it in the MultiMAN folder.