I use a lot of sound effects in my podcasting, and they all have to be royalty free and available for use. There's a term for music and sound that can be used in podcasts without fear of copyright claims: podsafe.
I thought I'd gather together a few of the places I get my podsafe music and sounds from.
Almost all of my sound effects (at time of posting this) come from Freesound.org which is a collective of exactly what it sounds like: Free to use sounds. There's a real range of quality, and you have to be a little choosy sometimes, but people who post their sounds on Freesound are generally happy to be credited and to know how their sound is being used.
Before Freesound, I used FreeSFX.co.uk which was good for the occasional sound. Unfortunately, their search bar is buggy and returns an error for any search terms with more than one word.
For music, I'll always turn first to Incompetech, which is the work of just one man: Kevin MacLeod. Brilliantly, anyone can make use of any of his music in any way, shape or form, as long as he's credited. That's why every single Pocket Fiction post has his Creative Commons attribution at the bottom of it.
Creative Commons has a search that I use often for pictures (in fact, that I've used for the pictures for this blog post!) but it also has a Wikimedia Commons search option, to find sounds. Incredibly useful.
And lastly, I use Garageband for all my podcast editing. Garageband comes with a load of free-to-use music. It's actually really surprising how often I hear something from the Garageband library on a podcast.