Oddly in most cases, you have to buy something that'll record your voice. VersionTracker has a nice list of programs you can check out here. I would recommend though that if your looking to buy something, upgrade your QuickTime player to QuickTime Pro. It gives you voice recording abilities as well as tons of very useful features.
Sorry to interrupt, but can anyone tell me whether the Power Mac G5 even has an internal mic? I know the iMac G5 does, but it doesn't show up in my Power Mac's sound input list in sound preferences. Thanks
Tower case Macs don't have a microphone.
I use "Audiocorder" with a very cheap external (Chinese manufactured) desk mike, connected via an iMic USB adapter.