Thank you for the suggestion TyrOvC, but using Caster is what I planned to do all along.
I've accomplished about one third of what I want to do with the source code (that includes implementing Caster audio), but I'm now having issues with Game Maker 7 for Mac.
I'm using Game Maker Pro 7.5.87 and when I try to "Create Executable" it seems to work fine except when I try to run the resulting app. The Spelunky logo pops up and then I get a black screen and no sound. This happens with Spelunky 1.0, 1.1, 1.2 and my modified version of the 1.2 source. I can exit the game with the "Esc" and "F10" key combo or "F4" and selecting "Quit", so the game logic is still working to some degree...
Here's the weird part. All those same versions of the game run fine (except for the Supersound errors) when I use "Run the game" from within Game Maker. I want to implement more changes in the code, but without the ability to redistribute a working executable, I don't see much point.
If I can't find any help here then my next step will be to contact L about his Mac port of Spelunky 1.0: http://mossmouth.com/forums/index.php?topic=2157.0
Thought I'd give the forum a go before I PM him. Worst case scenario, I'll just share the modified source as is and walk away.
Please help if you can, because I don't want to give up before I've barely begun.