When using music in a presentation, please remember to abide by copyright rules.

If the work is a published work, then the standard is to use only 30 seconds of the song, or no more than 10% if the song is short.

The sites below contain copyright free or Creative Commons music that the authors are allowing the public to use.  Do read the terms of agreement for each site when using free music.

Internet Archive

Opsounds --search by genre (more info about using the music here)

Creative Commons CC mixter 

Chris Worth Productions

Royalty Free Music.com

Podsafe Audio

Joyce Valenza's list of Copyright free Music sites: 

Copyright and Fair Use for Schools for Print and Multimedia  (What teachers and students can and cannot use due to copyright laws)