There's two solutions I've found:
1) Here is a sample from my Description.ext for the sound shilkadead. The key is the "db + 200" part halfway down. That increases the overall volume level. However I have found that doesn't always, work, so see solution 2
class shilkadead
{
// Name to display in mission editor
name = "shilkadead";
// Sound path, volume, pitch
sound[] = {\sound\shilkadead.ogg, db + 200, 1.0};
// Sound title text (set to no text)
titles[] = {0, "Well that's done! Now for the Shilka!"};
};
2) Download Goldwave sound editing software, (
www.goldwave.com) and then click on Effect on the drop down menus. There you can change volume, I've had to up my .wav files to +12 or so to get enough volume for in game. Then save the new .wav file (you should see the graph widen out to reflect the increased sound volume). After that you can turn it into an .ogg file for the game.