MIDI files usually only contain instructions that trigger sounds played by a synthesizer. On a personal computer the sounds are commonly generated from digital samples of musical instruments stored on its sound card or in large files. There can be remarkable differences in the sound produced by a MIDI file, depending on the machine on which it is played and the instrument samples it uses.
The following problems might occur when playing MIDI files:
The machine on which you play the file must have both the software and hardware required to play MIDI files.
Your sound card mixer playback settings might need to be changed. You can enable and unmute all playback devices, and then increase the volume of all of them, to ensure that your sound card sends MIDI audio to its output.
MIDI files encoded with a more recent version of the MIDI standard might not be correctly rendered if your machine doesn't support that version of the standard.
Some hardware manufacturers produce proprietary extensions to the general MIDI standard that are not rendered by all MIDI players. Such hardware-specific extensions are to be avoided in MIDI files uploaded to Wikipedia.
If your sound card does not support MIDI, TiMidity is able to play these files, or convert them to other sound formats; also VLC media player is a cross-platform media player that can play MIDI files.