sound quality deterioration

After making a recording on my Yamaha P-80 digital piano and saving it as a MIDI file to my hard drive, I opened it in Windows and played it on my computer, through headphones. The sound quality I hear on my computer is more “tinny” than either: (1) the sound quality of other digital pianos on the Internet, or (2) the sound quality of the Yamaha P-80 itself.

Is someone able to suggest why this is so or if it’s possible to improve the sound of the MIDI file?

MIDI is not audio.
A MIDI file is a set of instructions that tell a synthesizer what notes to play, which sounds to use, when to play them and how loud.
When you play a MIDI file in Windows Media Player, these instructions are played through the software synthesizer that is built into Windows Media Player.
When you play on your Yamaha P-80 digital piano, the sound is produced by the hardware synthesizer built into the piano. The Yamaha P-80 digital piano synthesizer is probably a lot better than the synthesizer that is built into Windows Media Player.

If you want to record the “sound” of the Yamaha P-80 digital piano, you will need to make an “audio” connection from the keyboard to the computer. The best way to do that (if available on the equipment) is to use an audio cable to connect the “Line/Aux out” from the piano to the Line input on the computer.

If your Piano does not have any audio outputs other than a headphone out, you could use the headphone output.
If your computer does not have a “Line level” input then you may be able to use a “Mic” input but it may be horribly distorted and mono.

I use the headphone out. That’s how I created all the piano and organ test clips on by web site.

If you have a MIDI manager or sequencer software, you can get your MIDI instructions to play on any instrument. You’re complaining about different piano sounds, but I’m playing my piano solo on a sax or a flute. As above, MIDI is the instructions of what keys to press, not the actual sound.