The MPU401 was one of the first PC midi interfaces. It was an ISA plug in card.

When midi was added to the game port, the midi input and output devices were also
referred to as MPU401, I'm guessing it followed the standard that Roland had set.

What are you using as a midi interface? Is it through a game port on the motherboard?
Or is it a game port on a plug in card? Or something else?

The old Windows98 PC I built had a game port on the motherboard that I was using for
midi. I can't remember though if the driver was one of the motherboard drivers or was
included in Windows98.

In the control panel, under Audio, under Sound and Audio Devices, is the MPU401 listed?
There's a section there for MIDI music playback.