Actually I also searched this on Google and the same problem is all over the place with many suggestions. Since you didn't tell us any solutions you've already tried, i'm just going to list as many as i can, hopefully one will help you and/or others as well.
Suggestions to try:
Reduce the FPS.
Change the default format for all playback devices and recording devices to 44100 Hz instead of 48000 Hz.
Use something else to record your voice. I suggest Mumble, with TeamSpeak 3 in a very close second.
Re-install Video Card drivers, Sound Card Drivers, Chip-Set Drivers
If you've overclocked your CPU or the front side bus, downclock it back to normal.
Issues were caused by the EPU-Engine. Uninstall it if you have it.
Try to record other games, see if you get the same problem, it might only be a certain game that causes the lag mentioned.
Don't try to play the raw Fraps file. Encode it then evaluate it.
Get or Use a USB Headset.
Use a different sound card, or set windows power properties to High Performance
Update your BIOS, or re-flash it
You can try to use other programs to record the audio to solve the issue, suggested software:
- Audacity
- PlayClaw
- HyperCam 3
I would personally never use fraps cause it records in raw format and normally have to encode the files later, the large disk space that gets acquired so quickly may cause the the audio to become out of sync (was suggested, but not confirmed) but makes sense to me.
That being said, if you're looking for a THIS IS YOUR SOLUTION, type of answer, according to everything I've personally read, the admins at fraps, don't know how to solve the problem either, so i doubt that's going to happen without trial and error.