Audacity is a free open source audio editor, which is good enough to help produce AV sound tracks.
Audacity is available here - http://audacity.sourceforge.net/
LAME codec (to allow exporting an MP3 file is available here) - http://lame.buanzo.com.ar/
How to Record using Audacity
Edit Sound with Audacity
Mixing Voice Recording and Music
This is excellent for an idea as to how to mix music and voice for an AV slide show.
Export as an MP3