Sumtingwong wrote:when I convert the track to mono using "Tracks -> Stereo Track to Mono", it gives me no file size savings and no choice under the .mp3 export options to save as a mono file.
When you export a project that is all mono, the exported file is automatically mono.
When exporting as 128 kbps MP3, the resulting file size will be the same, whether you export as mono or stereo, because there will be 128 kb of data for every 1 second of audio. The difference is that if you export a stereo track as 128 kbps MP3, those 128 k bits of data are shared between the left and right audio channels. If you export a mono track as 128 kbps MP3, all 128 k bits of data are used for that one (mono) channel, producing higher sound quality.
128 kbps mono will produce excellent results with very little loss of sound quality.
To make the file smaller, use a lower MP3 setting. 64 kbps will produce sound quality on a mono file that is almost as good as 128 kbps for a stereo file, but only half the size (because there are now only 64 kb of data for each second of audio rather than 128).
For speech, 64 kbps mono is likely to be more than adequate, 128 kbps mono will be near perfect.