Have you made sure your recording settings are the same in Fraps?
Have you pranked around with the settings when you right click the speaker?

Have you installed anything lately that could have effected your sound settings?

To be sure, I would check your settings after right clicking the speaker at the bottom right and make sure everything is still set right.
Installing certain programs will sometimes lead to that program changing your audio settings to better that program.

I don't how to fix your problem, but for a minute I thought it said:

"Fap Issues"

Sorry. (>.<)

Sometimes something similar happens to me where i'd import a video recording and there wouldn't be any video and only sound. Basically what fixes that problem for me is just simply save whatever work you've already done and restart vegas and reload the project file. In most cases it fixes it for me, do it a few more times if it doesn't fix itself since it usually rights itself after a few tries. Otherwise what you can do is do a cheap work around and just transcode the recording file to a similar format and quality and just import that into vegas.

What if you try it with Windows Live Movie Maker ? If you have sound there .. then it's something with Sony Vegas I guess. :s

Open it with Media Player and see if it has audio at all.if it was okay then there is something wrong with your Vegas.

If you try to only record gameplay sound (not your voice) you can record this separately by going to sound options and go on the record tab right click and "Show Disabled Devices" and enable stereo mix and make it default recording device. Use Audacity or Wavosaur or whatever to record the sound while you use fraps to record the video.
Then import the sound and video into vegas and you should be good to go!