I would have to concur.
I run on LX and wine, therefore I use LX folder structure. Although my testing was slightly different. I was running as FG server/hosting the session and therefore had the DOE:sound extension loaded, and all my local soundfiles use the LX folder structure. As the host, in my options I disabled sound. A player/client that runs FG on windows was connected to my instance, and he had sound enabled. Sounds would trigger and play as normal, but not on client side. With him enabling sound, and a sound event occurred in chat window, it would trigger the play of my local soundfile (because I was the host and I had all the chatstring triggers configured on my end, therefore they pointed to my localfiles).
I never tried to figure out a way to get a soundfile on their computer to work. But in essence I don't think it can. Even with wine emulating windows folder structures, it's not really the same.
Unless someone REALLY knows MAC well and has done experimenting, I don't think you'll find a quick answer. I suspect you might need to do some serious digging to figure things out, and let everyone know what you learned.
For me, I can only point out what i've found using LX and Windows ... which doesn't help your situation.

