http://audacity.sourceforge.net/manual-1.2/tutoria l_common_ed_1.html

Audacity makes it very easy to mix two different sounds together.

Open one sound (for example, the background music).
Select Import Audio... from the Project menu and open the other sound (for example, the voiceover).
Listen to your sound using the Play button. Audacity automatically mixes them together.
Choose the Time Shift tool and adjust the position of one track or the other until they're synchronized the way you want them. You can even move tracks around while they're playing.
If you hear clipping which wasn't present in either of the original files, it means that the combined volume of the two tracks is too loud. Use the gain controls on the tracks to reduce the volumes until you don't hear clipping anymore.
Export as a WAV or MP3 file.