Improved sounds

Added:
*Menu/Selection beeps
*PB6 now toggles volume in similar manner to speed setting toggle (increment to max then decrement to min)
*PB7 now mutes sound

Next:
*Add ability to mute from pause
*Add more sounds (ie sound when enter/exit pause, and possibly game over tone)
*Get on the LCD (http://www.8051projects.net/lcd-interfacing/lcd-programming.php)