First off, go download and install Audacity. It's free, and it's a great piece of audio editing software.
Then open the sound file you want. In my case it's an episode of The Best Podcast In The Universe.
Now, you want to mix it down to mono, like this:
The sample rate is changed in the lower left corner. Set it to 22050.
Then transfer the file to your raspberry Pi and start your own radio channel