Command & Conquer 3: Tiberium Wars
It sounds like a driver problem or a soundchip that doesn't support enough audio channels. If it's a laptop or on-motherboard sound codec...
If you are using on-board audio (instead of a soundcard) try checking the developer's web site for updates, I've come across this problem with people using the AC'97 on-board codec. And secondly, go into the control panel.. Sound options; Volume tab; Advanced tab; and check to see if the sampe rate conversion is at "Best" and Hardware acceleration is at "Full", you may have to tweak those to find the right balance.
Also make sure in the Audio tab you have the proper playback device set and you may have to check use only default devices. Some Soundblaster drivers will randomly "lose" the default device if it sees an on-board sound chip because the cmos setting somehow changed.
Worst case scenario, max the volumes inside the game options, but don't change the volume if there's one in the tasktray.
Absolute worst-case scenario: Ditch whatever it is and get a brand new Creative X-Fi card or even a SB Audigy 2 series card, the Audigy 2ZS is on two of my lan boxes and no problems
You updated all your drivers?