You'll have to record the audio separately - this is a freeware option, there are many, many recording programmes that provide the option for voice recording whilst screen recording. The simple vid below demos the QT option, whilst the following reviews it in more depth - recommending that you use a paid-for alternative! I did look at another freeware option, Jing, but found it records in swf (Flash) format which requires 2-step conversion; you can convert your QT file if you need to using FLV Crunch (again: freeware).
Here's Apple's own short guide:
There's a comprehensive Wiki on the different screen recording software: http://en.wikipedia.org/wiki/Comparison_of_screencasting_softwareVid1: Simple demo of using QT recorder
Vid2: MacMost review of QT 10 recording
Any more suggestions, please add as a comment below