Changing the byte at $000de7 from 4 to 0 will turn off the in-game music for the first stage
maincpu.mb@de7=0
There's lookup data at $000de6 consisting of ten words, I'm not familiar with this game and have never played it before but I presume there are 10 stages to the game in total and this lookup data contains the index of the tune that will be played for that stage.
If that's the case then simply change the low order byte of each of those 10 words to zero to turn off the in-game music for all stages.