The middleware issue you needed to write is going to be open sourced, right? ;-)
The middleware is about the sound, isn't it? You wouldn't be the first studio experiecing exactly that kind of problem.
@Raze: Sorry for not being so clear about that point. I didn't mean you should have one code base for all versions of the game. Of course you do need different branches if you have a release with a lot of new functionality being added. What I ment is that it's more handy to have one branch that should be able to be compiled on every OS from the very beginning of the developement progress. That also makes sure releases could be at the same date. In this case also MacOS and Linux users are happy to buy early access etc. as well! Extra money for you.