Mono: Use this setting for mono sound files, or to generate mono MP3 files which come from a stereo source. Stereo: Takes the left and right channels of the original audio track and encodes them separately; e.g.: a 128 kbps Stereo encoding is actually two 64 kbps encodings (one for the left channel, one for the right) sandwiched together. Joint-stereo: Rather than encoding the channels separately, joint-stereo encoding tries to find the similarities and differences between the two channels. This method offers improved stereo sound quality. Force stereo: Same as mono, except it makes sure the mono sound is going through both left and right channels.