Various EMI sound fixes

1. fix volume calculations in SoundTrack::updatePosition()
- avoid integer division by forcing floating point operations to avoid a floating point division by zero (happens in swamp when heading north with the raft)

2. don't abort when LUA code calls PlayLoadedSound with nil object
- avoids an abort() of the game when entering the bait shop after the ants are neither in the room nor on the prosthetic hand