https://forum.pjrc.com/threads/27409-Play-RAW-from-Serial-Flash?p=77666#post77666

While changing everything else I also modified most things regarding
gain or volume from using 0 to 100 (or -100 thru 0 to 100) to using 1
(or -1 to 1) with only one exception I can think of right now in the
autoVolumeControl(..); - I modified all examples I found .volume or
similar in to suit.

I found opportunity to write these changes, I did not have a chance to
test much more extensively than just compiling it. I will have a chance
to test them by about 24 hours time.

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?

Simplistic visual peak meter just outputs to terminal.