The 'tone' filter examples are both using calcBiquad(..); but one uses
AudioFilterBiquad and other uses SGTL5000 PEQ filters. Had included code
for volume level control optionally and decided to drop it from both.

Changed method for setting level/gains for left & right from
'item_left(..); item_right(..);' style to 'volume(left,right)' style
instead.

PlayMidiTones example modified to work with ramping.