Oh, you wanted the how as well ?
Go to iChat menu > Preferences > Alerts
The top item is a drop down of 18 different Actions the app can make that can have Sound and Dock icon bounces, AppleScripts and Speech added to.
In the case of text Chats you might want to have something for the Chat Invitation as well as Message received
The next item is also a drop down to select the sound to use.
The top part are the Sounds in ichat
The next are the System Sounds seen in System Preferences > Sound > Sound Effects as the Alert sounds there.
The final section is the Users Sounds
These are held in Hard Drive Name/Users/(your account)/Library/Sounds (this Library is hidden in Lion and Mountain Lion).
The sounds are played in iChat when testing or setting up.
In everyday use they are played as if System Alerts and at the volume of System Preferences > Sound > Sound effects moderated by the Master Volume.
Generally speaking iChat sounds are "softer" and quieter than other Alerts sounds such as Mail "New Mail" sound.
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously