there is a specific adaptor loom which fixes the speaker volume.
my car has bose and this morning i am fitting (after weeks of messing about) my alpine x100 head unit.
you need these:
Connects2 CT53-AU04 - Audi A2, A3, A4, A6, A8, TT Active System Interface Amplified Adaptor Lead CT53-AU04
PC5-90 AUDI A2 A3 A4 AMPLIFIED ISO - DIN AERIAL REPLACEMENT ADAPTOR MAST ANTENNA 150713509461
top one is the head unit wiring. the second one is a powered aerial adapter which powers up the radio aerials embedded in the rear screen. without this, the radio signal will be extremely weak or not receive at all.
as you dont have bose you probably/definitely dont need the CT53-AU04 adapter and your generic one will be OK. but deffo get the PC5-90
