For the time being, most computer-based recorders record in some variant of the WAV format. If the WAV format is PCM16, speexenc (the Speex encoder software) can handle it. For other WAV variants, you may need a sound file converter to convert it to PCM16 format first. Do not be alarmed by the large size of the PCM16 WAV file, it will be about 30 times the size of the equivalent Speex file.
If you use Linux, you can use sndfile-convert to translate from one file format to another. This utility program is rather versatile.
Once you have a file that is in PCM16 WAV format, you can use speexenc to encode it to Speex format. Although speexenc has much flexibility in terms of controling the outcome, the easiest method to control the output is via the -quality option. The default is ``quality 8'', but that produces a file that averages at more than 15 kbps.
From experiments, a setting of ``quality 2'' is sufficient for lecture purposes. This yields an average bit rate of less than 8kbps.