When you stated you have the flashing LED on your UM-one, was that while you were in Dosbox or just in Windows?
In Dosbox there are two basic things you need to do to get Midi working, especially when changing out to a new computer:
1. - Load up dosbox and as soon as you get to your prompt type:
That should give you a small list of midi devices that dosbox is able to detect. You will likely show the Windows GS soundbank that is standard as well as the UM-one. Make note of which device number the UM-one is.
2. Edit your dosbox.conf file so that in the midi section you have these specified:
Midi config = "#" of your um-one device
That really should be it. There are a few other things in the dosbox config as well you can adjust like the midi mode being intelligent..etc and the Midi device I usually leave as Win32 if I remember correctly. But the Config line is the most important so that dosbox knows which midi device to use. If I don't specify this, then I either get nothing or I usually end up with the windows default GS bank instead.
Midi Modules: MT-32 (OLD), MT-200, MT-90, SD-20