Add new header files to Global.h
Add audio ISR to ISR_vectors

This is a slightly modified ISR from module 1.
Add sound functions to Song

The Song struct now holds onto a sound.  The sound can be managed using
the Song functions.
Add an AudioDeviceController

This controller initializes the alt_up_audio_dev, handles the av config
setup, and registers the audio ISR.
Add a Sound struct to hold the raw sound data

The Sound struct holds the raw sound data and allows playing, pausing,
looping, and eventually more effects to be added to a specific sound.
Add a SoundMixer class

The SoundMixer handles the reading of the currently playing sounds.  It
checks to see which songs from the database are playing, and combines
these sounds together.  The Mixer allows for global sound management and
effects on multiple sounds.