You’ve heard these all your life on the radio and on television. They’ve been a staple of broadcasting for generations. When someone makes a funny mistake on the air, for example, it can be even funnier if it’s immediately followed by a judiciously placed "Boing!" sound. Other common sound effects include
Various bodily functions (use your imagination)
Explosions or gunfire
Most of the time, these kinds of sound effects are used for humorous purposes. Occasionally, though, they can be used to enhance a serious narrative or to emphasize a point.
In the early days of podcasting (back in late 2004!), adding sound effects was a laborious process. Unlike DJs on the radio who had machines specifically designed for sound effects, there was simply no convenient way for most podcasters to add sound effects into a show as it was being recorded. As a result, they usually opted to not use them rather than investing the time it took to add them to a show in post-production.
Today’s podcasting software makes it much easier to work with sound effects and to add them into your podcast on the fly.
Sound effects are available from a variety of sources. Some podcasters record their own. There are also websites that offer bundles of sound effects you can purchase online or on CD. This website is a good place to start: http://www.a1freesoundeffects.com/radio.html.
You’ll find dozens of sound effects that are available to you free for non-commercial use.