sax supports quite a few other manipulations. I ended up with a commandline something like this:
sox INPUT -t raw -c 1 -r 8000 -w -v 0.25 OUTPUT resample -ql
The "-v 0.25" between INPUT and OUTPUT will reduce the volume to 25%.
The "resample -ql" after OUTPUT should make the rate reduction sound better.
There are a lot of other things you can; see the sox manpage.