Full CD-quality sound files are very large. In most cases, we won't need the full quality, and would like to reduce the size of the files.
sox can resample a sound file to a different rate. e.g.:
sox track10.wav -r 16000 newtrack10.wav
will convert the input file (track10.wav) to one at 16 kHz (newtrack10.wav). If the input file was at 44.1 kHz, the new file will be only 36% as big.
It can also convert a stereo file to mono - this will cut the size in half. e.g.:
sox track10.wav -r 16000 -c 1 newtrack10.wav
both reduces the sample rate to 16 kHz, and the number of channels to 1 (monaural).
sox is also able to convert between different audio file formats, and to apply many different filters and special effects to a file. e.g.:
sox quack.wav quack2.wav pitch 200 echo 1 .5 200 .4
applies a pitch-shift and an echo effect to the input file quack.wav.
See the web-site or man sox for more information on the available options.