Now this part may get tricky so stay with me.

First thing you want to do is visit the usermap's webpage on the Steam Workshop. (If you don't already have it enabled, go to your Steam tab in the top left above the back arrow. Go to settings, then interface, and make sure that the check box labeled "Display Steam URL address bar when available" is checked so that you can see the URL under the STORE tab.)

Now, on the usermap's webpage look at the end of the URL for a 9-digit number. This is the usermap's ID number and you're gonna want to remember it.

This may differ for some people, but go to your Steam directory. For me it's C:\Program Files (x86)\Steam, but it may be different for others. Go to this directory EXACTLY: (Without quotes of course)

"Steam\SteamApps\common\Duke Nukem 3D\gameroot\workshop\maps\123456789"

123456789 represents the ID number of the usermap you're looking for. Once you find the correct folder, there should be a zip file with the map's name on it. Click on it and find the OGG file within the folder. Take that OGG file and put it onto your desktop for now, we're gonna need it later.

Now go to this directory:

"Steam\SteamApps\common\Duke Nukem 3D\gameroot\music\usermaps"

Now that we are in the usermap music folder, take that OGG file from your desktop and drop it in there. Now it's all done! Launch Duke Nukem 3D, go to the usermap tab, if you're able to get to it without crashing the game, and play the map. You should hear the music now!

Hope this helped you all!