Here is the explanation about "MMI low" aka Concert/Chorus/Symphony Radio aka Audi Information System which ships in A5/S5s without satnav. It is not real MMI! It is different, see here:
Will the real MMI please stand up? Difference between standard radio Concert/Symphony and the optional DVD navigation's MMI controls in B8 platform Audi A4 and A5
http://www.geocities.com/a6retrofit/articles/a4a5realmmi.html
Retrofitting MMI/satnav is possible but a heck of a lot work, see here:
Notes on retrofitting MMI High (Audi A6/A4 B8/A5/Q7)
http://www.geocities.com/a6retrofit/articles/mmihighenglish.html
As for retrofitting AMI, notes and part numbers and links here, note that AMI is different depending on if you have MMI/satnav or not. AMI MOST for MMI, AMI CAN for the radios. If you don't have MMI/satnav, AMI CAN for the radios is compatible with radios starting model year 2009, that is build week 22/2008 onwards. Car build before that, you need a new radio.
AMI (Audi Music Interface) part numbers / Notes on retrofitting AMI (A4/A5/A6)
http://www.geocities.com/a6retrofit/articles/a4a5amiparts.html
Related thread cocerning updating the radio for AMI CAN:
Part # Request: Audi Symphony III radio
http://www.a5oc.com/forums/showthread.php/part-request-audi-7074/index.html
AMI does not allow you to watch video. If you have 2G MMI/satnav (prior to build Nov/2008), you need to retrofit some third party solution that hijacks the MMI screen or factory TV which containcs aux-in for video to watch video (you will need full MMI/satnav in A5 to retrofit factory TV). Instructions here:
Hybrid TV module installation Audi A6/A4 B8/A5 (relevant for Q7/A8 too)
http://www.geocities.com/a6retrofit/articles/a6a4a5hybridtv.html
Notes on retrofitting TV module Audi A6/Q7/A8 (goes for A5 too)
http://www.geocities.com/a6retrofit/articles/a6tvmodule.html
As always, this and a lot more retrofit stuff for Audis here:
http://www.geocities.com/a6retrofit/
Audi ships MMI 3G with the A5/S5 now, starting late 2008. Things are different for that, so these instructions apply to MMI 2G and the current Concert/Chorus/Symphony radios only, although in principle MMI 3G is a lot like MMI 2G.