I ran into this too, as described here: http://www.fmod.org/forum/viewtopic.php?f=21&t=18770#p59277.
Use Channel::setVolumeRamp(false) to fix it. I reeeally think this behavior should not be enabled on channels by default. 
I ran into this too, as described here: http://www.fmod.org/forum/viewtopic.php?f=21&t=18770#p59277.
Use Channel::setVolumeRamp(false) to fix it. I reeeally think this behavior should not be enabled on channels by default. 