I haven't seen anyone else mention this problem, so maybe it's just me. On my machine, the volume controls, say for Effects, are very coarse. There are only 6 different levels. What's worse is that they aren't deterministic; i.e. they don't always do the same thing despite the same setting. Example: say I'm in a tank with the motor running. Setting the Effects volume to zero sometimes sets the engine volume to zero, sometimes to faint. However I set it, except for Max, the sound level isn't always the same for the same setting. Or, if the volume is set to say 1/4, I sometimes don't hear the engine or tracks as I start moving. Then a bit later as I'm still moving, I'll suddenly start hearing the tracks, but still not the engine. Other times, with the same volume setting, I'll hear both. This is with no other sounds, like vox or external tanks/planes, interfering.
(I normally run with the engine and tracks, i.e. the "Effects", at lower volumes so that I can clearly hear them, but not loud enough to damage my hearing with hours and hours of constant loud engine drone.)
Win XP SP3, Realtek HD Audio onboard sound. I've always had this problem with all AH versions using the Miles Sound System.