Don't worry too much about the airbag light. If the shop simply removed one of the seats with either the battery still hooked or they turned the car on with the seat removed (and therefore the seat's harness was not plugged in) it'll set off the airbag light. It will NOT go away by itself, though. You have to clear the fault code. Either get the shop to do it or use Vag-Com - assuming that the seat was, in fact, removed.
If seats were not removed at any point than you better do as suggested and raise hell to get the audio place to fix their f**k-up.
I mean, having any fuse blown during a simple amp install is insane. That shop must be absolute garbage. I ripped my whole interior apart, installed multiple devices, etc, and I didn't have a single problem besides the airbag light (from removing the seat and then turning on the car), which I cleared with my Vag-Com.