Step 1: Obtain soundfont file (please refer to this guide to find sources of soundfont file)

Step 2: Install VirtualMIDISynth (Source)[coolsoft.altervista.org]

Step 3: Add soundfont to VirtualMIDISynth

Step 1: launch OpenTTD with the following debug launch option in steam: -d driver1


Step 2: In the Debug Console, look for "Detected DirectMusic ports" and take note of the port number of "VirtualMIDISynth #1"

In my case, the port number is port 1

Step 3: Set the MIDI device in Open TTD
In the steam command line arguements for openttd, add the following command line arguement -m dmusic:port=X (where X is the port number from step 2)

Step 3.1: Navigate to %UserProfile%\Documents\OpenTTD\
Step 3.2: Open openttd.cfg in notepad, or notepad++ or whatever alternative
Step 3.3: Use control+f to find the "musicdriver" field
Step 3.4: Set musicdriver field to "dmusic:port=X" (where X is the port number from step 2)

Step 3.5: Save and close file

Step 4: Launch the game and test if it sounds different (you can also look at debug console to see which device is selected)

(When finished, you may remove "-d driver1" from the steam launch options)

In case anyone who doesn't launch this form steam, most of these steps will be valid, but instead of setting the launch options in steam, you will set the launch options via your game shortcut