Fixed problem with tone_hi - tone_lo computation being done
with unsigned arithmetic, and thus always giving a positive result.

don't save r12

indeed..you're right, saving r12 seems to be not necessary.
i did some tests, even with gcc 5, all worked. but  i'm still only 99.9% sure .-)

Just learned that saving r0-r3 is superflues

http://stackoverflow.com/questions/261419/arm-to-c-calling-convention-registers-to-save

update

Remove debug Serial.print...

fixes #113

AudioControlWM8731::inputLevel(float n)

fixes #115

Add AK4558 CODEC support / Merge branch 'master' of https://github.com/MickMad/Audio