On this Windows 8.1 PC, double-clicking the File Explorer entry for a MIDI (.mid) file creates a Windows Media Player process, but that process creates an error box saying "Windows Media Player cannot play the file because there is a problem with your sound device. There might not be a sound device installed on your computer, it might be in use by another program or it might not be functioning properly."
But all was OK in October. Then, this Windows 8.1 PC played MIDI files OK: Double-clicking the File Explorer entry for a MIDI file created a Windows Media Player process which played it, using a General MIDI soundfont.
So something has gone wrong since then; how do I fix it?
As stated in this answer, the General MIDI soundfont file is called gm.dls. This PC has two copies of gm.dls (3361kB, 18 Jun 2013): one in C:\Windows\SysWOW64\drivers, and one in C:\Windows\System32\drivers.
The PC's sound system is Realtek.
Double-clicking on the File Explorer entry for an audio file (e.g. .mp3) works; a Windows Media Player process is created and successfully plays it.
I'd rather not use an app which reads a soundfont and a MIDI file and outputs an audio file. For one thing, it's one more step in the workflow, a step I used not to need. For another, the resulting audio file is much bigger than the MIDI file. It's more efficient on disk for the audio-player to convert the MIDI file to an audio stream on the fly. But if I must, I'd rather it be something easier to use than TiMIDIty, which was the only app I could find for the job, the last time I looked for one.
Garritan Personal Orchestra was mentioned in this answer but appears to be very expensive.