I do a lot of acapella recordings, and every time I record a second layer, the playback is slightly delayed for that layer. This happens with all layers after the first one, and can make for a lot of confusing sound, especially when I have 10 or more layers of voice at once.
You have to correct for the fact that a computer cannot lay down a recording instantly in real time. Please see http://manual.audacityteam.org/o/man/faq_recording_troubleshooting.html#sync .
Gale