With respect, MP3 for speech is perfectly acceptable at 128k, which is the default 'standard'. Once you go down to 56k, the twittering becomes more obvious.

The BBC (radio) store all the recorded station output as MP3, and all news items, even the music played and pre-recorded programmes, are all stored as MP3 (except where played direct from a CD).

It's worth bearing in mind that a 16 bit .WAV file will be about 10 times the size of the equivalent 128k .MP3 file, yet the difference in quality will be negligible to the average listener.

To be brutally honest, if you're prepared to accept the quality of a photograph (.JPG?) that is only around 400k in size, then you won't be the slightest bit bothered by a slightly lossy MP3, even at 56k!