
I've made a little hack this afternoon that allows me to use the music features of OpenTTD under Mac OS X and I share this with you here. Note that I have Mac OS X 10.3.3 (Panther) and this comes under the category of being a little messy. (You'll probably need at least Jaguar and the Java 1.4 update).
Basically, I've fooled my Mac into using the (handy) extmidi module for the Linux port.
There are two files in the hack, and no OpenTTD changes are needed (which is handy), and they are attached.
I have a shell script called 'timidity' which uses features of Bash to run a Java program which plays the MIDI files (note that the .gm files are simply MIDI files renamed - QuickTime and iTunes both recognise them *after* they have been renamed to .mid).
You need to put the script "timidity" into your /usr/bin folder, along with OpenTTDMidi.class, and make sure it is executable (you'll probably have to sudo to get access).
Then make sure the .gm TT files are in a gm folder inside the Open Transport Tycoon folder (created by the OpenTTD binary).
Then, to run OpenTTD, open a Terminal and change to the Open Transport Tycoon folder and type "Open\ Transport\ Tycoon.app\Contents\MacOS\ttd -m extmidi" and OpenTTD should pick up the music.
Let me know how it goes -- if it works for others I might post a .pkg installer file to automatically handle the installing.
Happy transporting.
Joshua King