The easy way would be to use the Macetech Shift VU Shield.
http://www.spikenzielabs.com/Catalog/index.php?main_page=product_info&cPath=26&products_id=108This shield is designed to takes audio input and send it to the Arduino as an analog value.
Another way would be to build your own audio to analog circuit, which would probably, in a basic form, only need a few capacitors.
In either case, I would use the audio from the VoiceShield's built on audio amp, it has more power than the audio line out so it will be easier to get an analog value from.