If you are interested in multi-track overdubbing with Audacity you may like to take a look at this new set of tutorials in the audacity manual:
http://manual.audacityteam.org/man/Tutorial_-_Recording_Multi-track_Overdubs
Many thanks to Greg Kozikowski (Koz) for the initial bench-testing and write-up - and thanks too to Ed and Steve for their assiduous editing.
WC