rogue_wraith:
My concept is to just run an audio line to the analog ins (not sure if this can be done directly or if I need to add components in between).
Absolutely. Audio directly into the Analog Input will destroy them. Line-out is such a small signal you'll at least need an amplifier. Also the A/D only samples about 10k per second, which is relatively slow for Audio.
Lakes:
The other way is use a MSGEQ7 chip
http://www.cmiyc.com/projects/msgeq7-simple-spectrum-analyzer/
I don't know if I would trust that guy's projects... 