HI,
The Audio Chat Enabled (Or possibly Microphone Enabled) in the Video Menu will be greyed out during Chats.
It is unlikely you would be turning it Off and selecting a different Mic is done somewhere else.
The key tends to be balance between System Preferences > Sound > Input, the Processor and the Bandwidth.
As a Start try turning Off Ambient Noise Reduction in the System Preferences > Sound > Input tab
Keep this panel open and start a chat and see if the slider moves (it has been known to in earlier OS/iChat combos)
Try the Bandwidth Limit in iChat at 500kbps for Video Chats (this is still fast enough for you to Host three and four way Video chats.
For 1-1 chats you could go as low as 200kbps.
Check that the computer is not excessively busy with another app.
The best place to check is the Activity Monitor (Applications/Utilities) and then click the CPU column to organise the this by CPU usage.
Anything running at about 30-40% is likely to effect things (that includes iChat itself.)
Also check what Bandwidth you are getting from your ISP
http://www.speedtest.net/
Technically the Connection Speed is the overall speed you get.
Bandwidth is the portion of that gets to your computer if there are phones, other computers and games consoles attached on the LAN.
The Speed test tells you the Bandwidth your computer can see.
In reality it should be close to what you pay for from the ISP less what are called overheads (the connection itself to the ISP takes some, the test itself also eats in to this).
If you are seriously down on speed try to eliminate other devices (Bandwidth is not shared evenly, is more first come first served but some apps and devices are better at grabbing their share.)
Check the router for devices connected and make sure that you can identify them all.
In your Hard Drive/Library/Preferences there is an Audio Folder.
(this is the Library you an see when you Open the Hard Drive icon)
IN the Audio folder there are at least two .plists for Audio Settings
Try deleting the com.apple.audio.SystemSettings.plist one.
Restart the computer after that as the .plist is Read on Start up. (if it is not there it gets recreated and it can hold on to the names of devices that have been connected in the past)
As you can see most of these are Stand alone things to try.
8:22 PM Saturday; May 26, 2012
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Lion 10.7.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.7.4),
"Limit the Logs to the Bits above Binary Images." No, Seriously