My scripting is rubbish but...
$1, $2 and $3 are to be your inputs, thse are variables interpreted from user input in the shell, so you'd run something like
soxmixmp3.sh <wav_file> <second_wav_file??) <mp3_file>
And it should convert the wavs to the MP3..
Try it but BACK UP THE WAVs!!!! The last line of the script tells it to remove the wavs when done.