Made a internal function to find the first empty audio channel to use

- Renamed enum ChannelTypes to enum AudioMixerChannelTypes
- Each channel now can be assigned a audio type (sound, music, etc)
- Each channel has a paused boolean state. Not done yet.
- In update loop: channels automatically get marked as available if sound is not playing and not paused
- Uncommented Debug.Log() stuff
Tester objects can now indicate what type of sound they are playing.