EDIT: Ninja'ed while testing it myself :P

Try running the setup from the GOG shortcut. When drag-and-droping SETUP.EXE onto the DOSBox exec you are not using the Warcraft .conf file but all default settings.

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In order to use the GOG installation modifying as little as possible, you need to:

1) Edit the dosbox_warcraft.conf file changing the line that says gus=false to gus=true

2) Copy (could also make a junction of, or hard-link) your ULTRASND dir to the base directory of the GOG installation (the one with WAR.EXE).

3) Run the game setup, by pressing 2 at the menu that appears when you run the default GOG created shortcut. Select Gravis Ultrasound for digitized sound card and/or music sound card. The setup program will detect the port and DMA settings automatically (differently than with SB). Then select Save settings and exit.

4) Run the game and check that both sounds and music are working. You will probably have to adjust the volume levels from the F10 menu in-game. The music sounds much louder at the default value with the GUS than with GenMIDI or SB16.

5) Enjoy!

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If you have or are planning to add more old games to your collection, I recommend you look into using a DOSBox frontend such as DBGL (https://dbgl.org/), and put all your game dirs (and ULTRASND) in a single directory that you mount as C: for all of them.