The internet is awash with websites that provide music and sound effects, but quite often using them in your own projects would breach copyright rules. Luckily there are many sites that now provide audio files under a creative commons licence. You can use these freely, as long as you attribute the original source. Sometimes you can even licence them to use commercially if you wish.
One of my favourites for sound effects to use in presentations or podcasts is Freesound. You need to register to download the clips, but it’s free to do so.
For music check out Jamendo – a community of free, legal and unlimited music published under Creative Commons licenses. There are many different musical styles available which would be great on podcasts.
Other sites that you might want to try include the following:
Incompetech: Search for music to use by genre or mood. Free to use as long as you give attribution. If you use a lot, send a small donation.
Sound Snap : 100,000 Sound effects and music loops to download. There is free content, but also premium content if you sign up and pay a fee.
Opsound.org : Described as “An experiment in applying the model of free software to music. Musicians and sound artists are invited to add their work to the Opsound pool using a copyleft license developed by Creative Commons”. Listeners are invited to download, share, remix, and reimagine.
ccMixter is a community music site featuring remixes licensed under creative commons where you can listen to, sample, mash-up, or interact with music in whatever way you want.
Free Loops : Free sound effects and loops to use in your own music remixes
A comprehensive list of sites and portals can be found on this wiki: http://copyrightfriendly.wikispaces.com/
You can also use the Creative Commons Search engine to find CC audio.