Update to 20180808 release.

byuu says:

This release fixes the XAudio 2.1 and WASAPI drivers on Windows, and
extends XAudio to support device selection (eg headphones, speakers,
monitor, etc.) It also adds DRC to XAudio, however it's not currently
working.

The code is courtesy of Talarubi, I just botched it somewhere upon
porting it to the newer version of ruby.

ruby/audio/audio.hpp

@@ -1,6 +1,8 @@
 struct Audio;
 
 struct AudioDriver {
+  enum class Format : uint { none, int16, int32, float32, float64 };
+
   AudioDriver(Audio& super) : super(super) {}
   virtual ~AudioDriver() = default;