Vorbis audio format is suitable for effective speech compression.

oggenc2 (command line tool) parameters for speech compression with best size/quality ratio:

smallest file, lower quality:
-q 0 --resample 22050 -S 1 --downmix --advanced-encode-option lowpass_frequency=6

small file, better quality:
-q 1 --resample 22050 -S 1 --downmix --advanced-encode-option lowpass_frequency=8