Hi,
iChat and Audio.
This has always been set by the System > Preferences > Sound > Input as far as the volumes you Buddies hear you. (Rather you turning this up has greater effect than them turning up their master volume)
This for a long time presumed there was only the Internal Mic or Line in on most Macs. (And that you weren't using Line-in despite showing both choices in the iChat Mic Drop down in iChat Preferences > Video Section)
Later this became an issue for Headsets particularly Blue Tooth ones.
These require that you set the System Preferences choice and volume first before then choosing them in iChat.
(If already set change both choices to something else.
Close iChat
Set the System Preferences choice to the Blue Tooth or Headset.
Now open iChat and set the choice there.)
In all cases since iChat 4 it has been possible to set the Volume level for a device (mic) in System Preferences and that it does not change you can use that selection in iChat but change the choice in system Preferences for other input/app combos. (i.e. once the volume is set for the device iChat uses the choice/headline device in System Preferences can be changedready for other apps)
I have never tested the Yahoo Beta in any build or sub version number for Audio Chats before today.
I tested between my G4/1Ghz Dual Processor running Leopard and my MacBook Pro running Snow Leoaprd.
Yahoo is allowed thogh both Mac Firewalls and the Modem/router combined device is doing UPnP.
Despite checking my Mic Settings in System Preferences on the G4 the Audio level seemed somehow to get turned down.
It stayed in the "right" place after I checked again.
Strangely the G4's older Yahoo Version 3.0.0 can Initiate an Audio Chat but the option is greyed out on the current Yahoo Version 3.0.1 that the MacBook Pro is running.
I will do some more digging.
However it would seem in your case that the Mic choice and Volume level would be the palce to start as it effects both apps.
7:47 PM Tuesday; August 16, 2011
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.8)
Mac OS X (10.6.8),
"Limit the Logs to the Bits above Binary Images." No, Seriously