I have same problem... P IV 3.4, ATI X800 XT PE 256mb, 3GB RAM, specially when lot of propellers sounds are around, or when you put the camera around a tanker in fire, fire sound eats the FPS, somo similar when there is a naval battle, in example in bismarck mission.
You can try using the windows sound recorder utility and reduce the quality of some problematic sounds from 44khz down to 22 or 16 Khz.
Do you use an onboard sound card ?
I do, may be the onboard sound card takes too much resources from the CPU ?