How do I open .brstm files on my mac?

I did a quick google search and it seems like a .brstm is a type of audio file. Have you tried opening it with VLC or Quicktime?

.brstm appears to be an audio file extracted from Wii/Gamecube games.

It seems difficult enough to find a Windows media utility for it that isn’t Winamp, let alone a Mac application!

Have you considered running Winamp under the Mac version of WINE? That would allow you to run Winamp, although I can’t guarantee that the .brstm plugin will work with it.

There is an app called Wii Transfer that may be able to understand that file, although it’s not exactly meant for that.