Speex dev - Jul 2007 - RE: a way restrict encoding sound volume

You can use the AGC (automatic gain control) in the preprocessor to
limit volume going into the encoder. That will not provide an absolute
hard limit, but will keep levels mostly "reasonable."

However, you also need to be careful that the capture device is set to a
sensitivity that is not too high. If the sound is clipped at the
analog-to-digital converter, you can not fix it algorithmically.