Audio on test branch

The first public release of Mini Metro's audio can be found on the test branch. We'll be pushing it to the main branch once it's had a bit of a thrash and any kinks have been ironed out. If you do check it out, please let us know if you encounter any problems! I think there may be some issues when playing with low frame rates.

Oh, and remember to enable audio in the options if it's disabled! :)