Well, either way it could require some guys to go back and edit their missions and/or scripts. However, I think it's better to unify the music playing system. All music played should be in the music channel and obey the same volume rules. It's just simpler that way in the long-run, IMO.
I'd like to understand this a bit better
The proposed change (or the scaling change The_E suggested) won't change the number of arguments to playMusic so existing scripts should continue to work. The difference would only be the volume level, which I believe will be overridden in most cases by the player anyway, either by changing the in-game volume (with the inherent annoyance of the volume resetting when new tracks start) or by changing the OS volume. In the case of the BtA Demo, all calls of the script set the volume to 100%. If that's indicative of the normal use case for playMusic then there should not need to be any change in missions/scripts (unless there's a need to rebalance the volume of the music files in order to have a specific impact on the player).
On the topic of a unified music playing system, underneath everything there currently isn't a single channel dedicated to music. Music is just assigned a free audio channel. i.e. you could have two music tracks playing at the same time (although you probably don't want to do that, unless you want a 2-part canon
). Changes in music volume are done by checking every in-use audio channel and changing the volume for all that have a "music type" (ASF_EVENTMUSIC or ASF_MENUMUSIC).
Anyway, am I correct in saying that the second part of your last statement means you prefer the change to just use the players music volume setting?