Metro will open in OSX, by starting up "Classic Mode"; however, it will not be able to use any audio driver except "built-in". Even "Sound Manager" devices won't work.

If you want to use an audio device or interface, you'll need to boot your Mac in to OS9, using the Startup Disk panel.

External midi communication will not work either, since Classic mode won't support OMS properly. Booting into OS9 will allow OMS functionality.