Implemented much of the public calls to AudioMixer's functionality

- Functions to set the pitch, priority, volume, pan, clip, audioType, and looping
- Functions to check if a channel is paused, muted, or playing
- Mute, Unmute, Pause, and Stop not tested.
- Test object now can set looping, pan, pitch, priority, and volume live in the demo scene.
Test object and example scene modified to accommodate new functionality.