You are not allowed to use copyright protected work unless you are using it in a Fair Use situation. Citing your source is not the same as getting written permission and is still breaking the law. While much online content out there is off limits for you to use there are plenty of places to find audio and other items that are available for use.

Copyright: The legal protection given to creators/ writers/ authors/ etc. which protects them against unauthorized use of their work. This includes sound, images, video and text. You may also want to learn more about Fair Use. This can be a little complicated, however, so whenever you are uncertain, it is best to avoid copyright infringement by using images and audio that you have created or that you have the rights to use. More information on the Copyright Law, and other legal issues can be learned about on the SPLC Media Law Presentations page or by visiting the JEA Press Right's Commission site. 

Terms to know: 

Royalty Free — Even Wikipedia defines Royalty Free as material (typically graphics such as stock photography, but also sound such as music loop samples) that may be used for profit, without paying royalties. Royalty-free media is usually acquired for a ‘one time only’ fee. Royalty Free does not necessarily mean it is FREE/ no cost. 

Public Domain — Music and lyrics published in 1922 or earlier are in the Public Domain in the United States. No one can claim ownership of a song in the public domain, therefore public domain songs may be used by anyone. (according to the Public Domain Information Project) 

Creative Commons — The Creative Commons (CC) is a non-profit organization that has released several copyright licenses known as Creative Commons licenses. Depending on the one the musician chooses, these licenses restrict only certain rights (or none) of the work. (According to Creative Commons)

Some suggested sources for music:
Garageband - create your own music! 
Royalty Free Music: http://www.royaltyfreemusic.com/ (scroll down to see the free music near bottom right)
Audio Feeds: http://audiofeeds.org/
Internet Audio Guy: http://www.internetaudioguy.com/iag/freemusic/freemusic.htm
Melody Loops: http://www.melodyloops.com/

Suggested sources for sound effects:
The Recordist: http://www.therecordist.com/pages/downloads.html
A1 Free Sound Effects: http://www.a1freesoundeffects.com/noflash.htm
Partners in Rhyme: http://www.partnersinrhyme.com/pir/PIRsfx.shtml
ONLY USE those from the ROYALTY FREE and PUBLIC DOMAIN sections