So I tried that and I still get the same error. I even tried some compatibility mode settings to no avail. I appreciate the suggestion though. =(
Hmm. The likely explanation then is that Munt is trying to write to a MIDI ID already taken by the X-Fi synthesisers. The installer is meant to jump to the next available MIDI ID but this probably hasn't been fully tested (most users will only have Microsoft's synthesiser).
Fortunately there's an easy way to free up the MIDI IDs:
1. Download and run Autoruns
(right click, run as administrator).
2. On the Codecs tab find the X-Fi synth entries.
3. Uncheck both X-Fi synth entries and close Autoruns.
Doing this will free up the MIDI IDs Munt tries to write to so it should be able to install.
Note that doing this will completely disable the X-Fi synthesisers. If you happen to use them for something there is a way to have both coexist with some further fiddling. Let me know if that's the case and I'll provide further instructions. :)
Anyway, hopefully this workaround will work. :)