timidity++Also, a brief explanation on what MIDI is, how it works, and why your request of something that "doesn't change anything of the sound" is invalid.
Like someone already said, MIDI is like music notes. It's a synthesizer file that contains notation for computer, instructions on what notes to play and with what samples.
Much like standard musical notation requires a musician to play the piece, MIDI requires rendering device to parse the file into waveform that is then sent to either computer's audio hardware, or just save it into a file that you can then play on more standard music players.
The reason why your request of something that doesn't change the sound of the MIDI is invalid is because there is no sound in the MIDI. It needs rendering equipment to become sound, and much like every musician's performance of same piece of music will sound different, every different rendering equipment will produce slightly different sound from same MIDI file. There is no "standard" MIDI environment, just ones that sound better and ones that sound worse.
Aside from renderer settings (surround, reverb, chorus etc.), the biggest thing that changes how the rendered midi file sounds like would be the soundfont.
Soundfonts are big files that contain audio samples of instruments. These instrument samples are used by the MIDI rendering device, based on instrument numbers. There is a huge lot of MIDI instruments and even more different sound fonts; the sound font provided with Microsoft Windows operating systems is, quite frankly, very small and crappy.
So, the most likely answer to your question is that you should use timidity++, in file output mode, and locate/download some good soundfonts and test which makes the MIDI file sound
the best it can, instead of expecting some nonexistent "standard" of how the midi file should sound like.
