A wild guess would be that it's a missing feature in IMuseDigital::playFtMusic(). These are the calls made as you move between rooms:

playFtMusic("radloop", 3, 28); // Room 21: Outside the trailer
playFtMusic("radloop", 3, 58); // Room 23: Inside the trailer
playFtMusic("radloop", 3, 127); // Room 25: In the basement

I think these are triggered by the call to soundKludge() in the entry script of each of these rooms:

Room 21: soundKludge([4096,6])
Room 23: soundKludge([4096,7])
Room 25: soundKludge([4096,8])