Adding sound and images to your podcast can make it more interesting, but only use material you know the rights for; copyright infringement can cost you up to $150,000 per use.
Anything in the public domain is free to use. You can identify materials in the public domain by the CC0 mark. Additionally, anything created prior to 1923 in the United States is in the public domain.
Sources for Public Domain music:
Sources for Public Domain sound effects:
Creative Commons licenses describe different rights that creators can assign to their works. Even if a work is not in the public domain you may be able to use it, depending on what rights are assigned to it. Internet Archive has a huge collection of audio files that you may be able to use depending on the license.