I am tired of seeing Serial.begin(9600); where 9600 is not going to be
used at all. Can Serial.begin(Serial.baud()); be acceptable?

Untested, will test it personally in next couple of days but sincerely
expect it will work so committing.

Todo: Test properly. Improve comments and possibly link to wiring
example(s).

modified an older 'detached' copy of the example source to try multiple
filters out and didn't notice the old comments till reviewing later.

Simplistic visual peak meter just outputs to terminal.

My manipulation of the contents of *data instead of the value of data
itself was foolish at best - forgot to leave the * off for my intent.

added last of dap 'bits' to control_sgtl5000.* and include an example
for bass enhance, need to make one for surround stuff.

Added
AudioControlSGTL5000::dap_avc(..);
AudioControlSGTL5000::dap_avc_enable(..);

SGTL5000_Specific/dap_avc_agc example sketch

Also added 'public domain...' line to all my examples.

https://github.com/PaulStoffregen/Audio/pull/7

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.