Not sure if the links above covered it , but I had copied this down from somewhere (I think the LSP forum).
"When you open Audacity, at the bottom of the screen, it says "project rate." Set that to 44100. Then on the left side of the screen, you'll see an X, then your file name, then a little arrow pointing down. Click the little arrow. This brings up a menu. In that menu select "set sample format." Set this to 16-bit pcm. Now go to "file" and select "export." Under "save as type:" select MP3. Click options and select, bit rate mode - constant. Quality should be 128 kbps, and channel mode should be stereo. Click ok, and save your file somewhere. "