Firesoul.. Chris' Mod of UAW for OP had a bug on the sfcmusic.txt file which I caught.. if they are using UAW mod, it is quite possible that when they are in that mod, the sounds will quit working for background music..
I have played OP + 4.0 streight and all my sounds are working correctly as far as i can tell.
As for Mod Chooser.. it copies any changed files that are in a mod to a separate directory and then when switching mod modes, it overwrites the existing game file.. no name changes are made at all... as such, using Mod Chooser, if your sounds.txt file did not have the txt extension, then if you launched mod Chooser (which searches for altered shiplist / fighterlist) and it finds a new mod.. it will compare the original file to the one in Taldren Original folder and if different size or date, then it will copy the file and use it exactly as it was in the game folder... IE, the file had to be bad before mod Chooser can copy it... So I doubt the problem would be the mod Chooser...
as for OP + 4.0, I'm looking in my OP directory under the sounds folder and the sounds.txt file is correct.. i have checked all mods in OP using mod Chooser just now to swap files and all sound.txt files check out...
the version of OP + 4.0 I am using is the exe with models... Is there a possibility that the problem may be in the no Models pack.. I dunno.. i didn't test that install of OP + 4.0 as of yet...
anyhow i hope that this helps.