It sounds to me like a syncing problem but hard to tell without code to test.
Things to try
1.Setting the sync rate lower may help SetSyncRate( 30, 0 ) // 30fps instead of 60 to save battery and allow the device to keep up
2.The Location of the Sync() command in your program may help
Sync, you may either use Sync or Update(), Render(), Swap() to manually sync. If you wish to have more control you can break this down further
and replace Render with Render2DBack, ClearDepthBuffer, Render3D, ClearDepthBuffer, Render2DFront but I'm unsure how or if these commands
will even help with music as I cant see a mention of that in the core help file.
Just some things to try sorry I couldn't be more help, but I have never noticed the music to fade in or out without coding it to do so.
GOODLUCK
fubar