LAME is a the best audio compressor for converting audio files to MP3 files. MP3 is inferior to Opus and not a ideal choice but you may want to convert files to MP3 anyway since quite a few (older) music player devices lack support for anything but MP3.
LAME is used from the command-line by simply typing:
lame [options] <infile> <outfile>
You may want to set more advanced options such as
-b for bitrate combined with
--vbr-bew for variable bitrate. MP3 sounds horrible at bitrates below 256, higher if you are an audiophile.
lame -V0 -h -b 320 --vbr-new somefile.wav newfile.mp3
See the the lame manual page for a complete list of encoding options.