I am a professional audio engineer, and work with a lot of different compression routines. I dont iChat
Could you record a bit from your microphone and post it here? I may not be the best person to help, but here are my thoughts.
My guess is a fault with the microphone. For giggles, try the headphones to see if it isn't software induced (correcting for feedback). If it sounds like your voice has been altered, perhaps it is the compression algorithm used to force your voice down the line. A and mu law compression algorithms (once used extensively by American and European telecom companies) do awful things to the audio. Most VOIP and voice chat programs automaticly adjust how audio is compressed. See if there is a preference for this. (Yahoo chat for PCs used to allow for diagnostics and altering of the routeens used.)
If youre using a modem, check your speed. If your sharing DSL, it may be your router or switch. If its an admind or intelligent switch, it might just be the need for you increase the bandwidth allocated to your network address. What exactly is your set up.
I'll post some links to some free sound routing software. This will help ferret out the problem. In the mean time, try these things, and look for an external headset mic (one of your friends must have an extra).
[EDIT] Look at this thread (my second post) for a link to SoundFlower and SoundFlowerBed- an audio routing utility.
https://forums.macrumors.com/threads/124734/. Also you might need Audacity, also linked there, to record the audio. Also Free.