I've added commands in ehBasic now so I can play music in the background.
An example:

SIDLOAD"delta.sid"
SIDPLAY :REM PLAY DEFAULT SUBSONG
SIDPLAY2 :REM PLAY SUBSONG 2
SIDSTOP :STOP PLAYING

The code to do this uses a separate 0-48KiB block for the music, so nothing of the stack, zeropage or other memory is affected in basic. Real multitasking :)