Mono vs. Stereo
Assuming that your source files were created in stereo, you have the choice to export them with both sound channels (stereo) or to combine them into one (mono).
For speech and most incidental sounds, mono is a good choice. Voices are rarely recorded in stereo because the sound comes from a single point (and it's more difficult to handle two microphones).
Music that you want to highlight and special 3D sound effects will always be more realistic in stereo. The only downside is that including both discreet sound channels will double the download time.
To determine whether an individual sound is converted to mono, select it and open the Properties dialog box. It's worth noting that MP3 compression does not allow for stereo with any bit rate less than 16Kbps. To force stereo output, choose a bit rate of 20Kbps or higher and uncheck the box marked Convert stereo to mono.