One way to create a "reverb" effect using MP3 Audio Mixer is to use the following trick:
- Load the sound.
- "Copy" the sound by right clicking and selecting "Copy" from the menu (or use the keyboard shortcut Ctrl+C)
- "Paste" the sound directly under the original, but this time move the sound slightly to the right (so that the copy of the sound would start less than a second after the original)
- Select "Pan" from the drop down menu on the tool bar.
- Pan the original sound entirely to the "left" speaker.
- Pan the copied sound entirely to the "right" speaker.
- Listen to it and adjust as necessary by moving the copied sound more to the left or to the right.
Alternatively, download and try Mixcraft! It's similar to MP3 Audio Mixer and has built in reverb and other effects processing!