I suggest randomizing the music in levels where more than one theme could apply to the level. This way, if you go to Lichenous Lair Shocking Synthesis, you might have either the slime music or the shock music playing. If you are in Devilish Drudgery Hiring Freeze you could have either the fiend music or the freeze music playing.

This way you won't hear the same music over and over when going though a shock, fire, freeze, or poison strata.