It would appear that AGS doesn't release MIDI device handles either when a game is closed down using Task Manager, or when a game is exited with Alt+F4. (Applications started to fail to open devices shortly after I did both of these, so I can't be too sure which it was.)
Since the only way to release the device is to reboot, this is quite irritating.
I'm on Win98, SE.
So it does work properly if you close the game down normally with the Quit option?
I can take a look at it, but I'm running Windows 2000 which doesn't lock the MIDI device to a single application so you don't get this problem; therefore it'll be difficult to debug.
I noticed the same thing on a Win2000 machine when I did a Ctrl+Alt+End.
I had to do this because for some reason the game that was running minimized, returning the desktop to the screen. It's not my machine so I can't say what was going on with the game minimizing. I just assumed there was something else running on that machine that made this happen????