This takes the audio in (AC-coupled) and amplifies it (inverted, but that won't matter) with a variable gain of up to about 5.5 (use R1 to adjust gain). It also centre-biases the signal around 2.5V (analogRead = 512 on the Arduino).
https://electronics.stackexchange.com/questions/185650/handling-audio-signals-on-a-5v-adc
