TiMidity++ is an open source MIDI to WAVE converter and player.
TiMidity++ is a software synthesizer. It can play MIDI files by converting
them into PCM waveform data; give it a MIDI data along with digital
instrument data files, then it synthesizes them in real-time, and plays.
It can not only play sounds, but also can save the generated waveforms
into hard disks as various audio file formats.
TiMidity++ is a free software, distributed under the terms of
GNU general public license.
example of converter:
timidity-snap.cli input.mid -Ow -o out.wav -c $SNAP/etc/timidity/freepats.cfg