Down-sampling a WAV file greatly, to reduce its size, will also noticeably reduce its quality. The MP3 format uses more advanced methods to significantly reduce the size of a sound file without major (human-detectable) loss in quality.
lame is a tool for converting WAV files to MP3 format.
A copy is in ~dave/bin/.
~dave/bin/lame bgmusic.wav bgmusic.mp3
will convert the input file bgmusic.wav to the MP3 file bgmusic.mp3.
Changing the bit-rate will allow you to further trade off quality versus output file size. The default bit-rate is 128 kbits/sec (i.e. one second of sound takes 128 kilobits, or 16 kilobytes; one minute of sound would take just under one megabyte).
~dave/bin/lame -b 32 bgmusic.wav bgmusic.mp3
will produce a 32 kbits/sec file.
The command mpg321 will play an MP3 file. e.g.:
mpg321 bgmusic.mp3