>>I would give a try with installing different codecs then.
>
>What do you mean by installing different codecs? Where should I get it?

codecs are programs that windows uses to decode audio/video - bad codec can result in bad audio or video.

Al posted a link that had some links to codec packs - I've used this one in the past with good results:
http://www.codecguide.com/download_kl.htm

The webex support site did say this:
Q. How can I improve the quality of the audio when using local recording?

A. If audio quality is poor or if there is no audio, make sure that:
You used the WebEx Audio Setup Wizard to test your volume.
If you are recording audio from a source other than a microphone, start the recorder and select the correct recording source.
If you hear “clicks and clacks” during playback, you may be using an older version of the WebEx Player. Re-install the player on your computer and try again.

Q. What audio compression and sampling rates does WebEx support? Which ones should I use?

A. WebEx supports the following audio recording formats:

G.723 (8 kHz) This is the default setting; it is optimized for streaming over the Internet.
PCM (8 kHz, 22 kHz, 44 kHz) This results in higher audio quality but much larger file sizes. This is recommended only for recordings that you distribute on a CD-ROM.

..so it looks like you should try to get codecs for G.723 (8 kHz)