Tapping into the CDC audio lines is easy enough and can be done to suit your needs
But the impedence is wrong, so you ideally need a matching circuit. Even I notice, despite being half deaf and completely tone deaf.
Depends what Audio Source your pumping into it, if its a proper Pre Amp Audio level then its not too far of the mark, so modern RCA type level should be ok if not a bit high, level wise, which is good in a way as it means you pad the input with a 2k to 10k Pot and try and keep the input resistance above 1k.
If your feeding the input from a iCrap, Phone, headphone output then its going to be all over the place, so more of a case of think of number, divide by 10, multiply by 2, then down the Pub to discuss with your Techie mates, Bobs your Uncle.