This is a step-by-step tutorial for conversion from an original music format, to an MP3 render for easier listening.

You can use Audacity for conversion from WAV to MP3.

MIDI - Open Audacity and open your MIDI in Windows Media Player, set Audacity to record from your output, and while recording, play the midi!

S3XMODIT/AMIGAMOD/<x>k - If you use OpenMPT for tracking, you can simply export as lossy. However, most trackers only export to WAV.
Use Audacity to convert to MP3.

OR! Use http://media.io to convert in one step!

NSF/NSF+: Use Famitracker to export your FTM to WAV, and convert. OR! Use NSFPlay or an online source to do it from an .nsf!

MARIOPAINT: Open the .SHO is MarioPants, export to .WAV, convert.

PANDA: Press F9 to export to WAV, and convert.

SUNVOX: Export to WAV, convert.

GAMEBOY/SNES/SEGA/COMMODORE 64/PC-ENGINE/YMU: In Deflemask; Options -> E WAV. Convert.

ZX BEEPER: In Beepola; Tools -> Compile to WAV file. Convert.

ADLIB: In Adlib Tracker II; Alt-F2 to save to module, convert with media.io

RENOISE: Export to MP3.

FSOUND:

CHANNELF: