How do I unmute during ichat on my mac book pro

In iChat 4 and upwards the Video and Audio streams are on the same port.
This would make it virtually impossible to block one without blocking the other.
Just to double check
System Preferences > Sound > Input
Ideally this should have the Mic chosen and the volume set to make the blue bar get to at least 1/2 way (3/4s is better). It should not move so far that it bangs into the right hand end.
This will cause the OS software to "Clip" the Audio to prevent feedback - which means no input is made and you will hear nothing.
Next.
iChat > Preferences > Video section.
If there are two Inputs showing in System Preferences he will have a drop down
It should list the same device.
The green bar below the Video pic should also move (Although less than the blue bar did/does)
Headsets
If he is now using a Headset.
Change the iChat Option to anything but the headset mic.
Do the same in System Preferences > Sound > Input.
Now set System Preferences > Sound > Input "First"
And iChat > Microphone Drop Down "Last"
He may need to do this for an Output choice if it appears in iChat and in the System Preferences > Sound > Output.
He needs to be concerned with with getting Sound In to iChat as that is what you hear.
It is easy to think that it is Output as it goes over the internet to the Buddy.
More.
The Video Menu of iChat
In here the Microphone Enabled should be ticked
Selecting this items toggles the tick on or Off (And the Mic On or Off)
I would also check the View Menu
In here make sure the Show Audio Status is ticked.
This will show Audio icons next to the Buddies (Particularly if you deselect Show Video Status)
Show Video should allow Audio but it will not hurt to check.
In fact I would toggle the Microphone Enabled and both Show Statuses to make the App write the info to the .plist
Then try an Audio Only chat
You can do this from the Buddies menu (Or right clicking a Buddy) or by having Show Video Status Off.
Yet more.
I would also open the Connection Doctor from the Video menu at both ends during a chat
This will report the stats.
If his Bitrate is not stable then his audio may be effected.
iChat will try to cope with changes by Buffering the Video which means dropping frames which then means the audio is dropped as it will not be in sync.
You can try Limiting his Bandwidth in the Video Section of the Preferences to 500kbps
This will smooth out large variances when using Extremely fast services that vary.
500kbps is more than enough for a 4 way Video chat as Host but is a useful tool in iChat 5
If his Framerate also varies it will be a sure sign that his Internet speed is varying.
It may pay for you to drop your to 500kbps as well as hs end may be struggling to deal with high data loads from your end.
As a Last resort try both end at 200kbps (Anything below this will not Video)
Change one end at a time and keep an eye on the Connection Doctor to see when the end that has changed has settled. The Buffering will smooth out the change.