Regulated audio input
Most OpAmp's and Digital Input's of Micro's now have Clamping Diodes to protect against minor transient Electrical Over Stress (EOS). However, to protect against larger sustained EOS, external Clamping Diodes are recommended. 
See TI's e2e below picture.



Where the ATmega328's Input High Voltage is VCC+0.5v. This indicates there is such an internal clamp. Where it can not sustain much stress above this. Additionally, a like Diode clamping pair should be on the Input to the Micro. 


EOS from the input is clamped by the first Diodes into the V+'s
Zener, in the above picture
Over amplification of the OpAmp's output would then be
clamped by a like diode pair on the input to the A2D.