I'd scan the relevant module to see what error codes are there, if any. I'm assuming that the SatNav and telephone sound comes from an independent speaker which maybe don't go through the amplifier, and if that's the case, it may be the amplifier itself. Also worth checking the relevant fuses. From what I can find, there appears to be three fuses related to the amplifier. They are located in the fuse panel in the boot, along the seat back on the right. The three fuses are F1-F3, right most column.
You could also try rebooting the MMI to see if that helps. The following video shows how to do it:
https://www.youtube.com/watch?v=aBYfRv1Sxa8
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2001 A8 D2 FL 3.7 Q - LPG Conversion, Engine: AKC, Gearbox:FBB, Colour:LY5X Aqua Blue pearl effect, Interior:WJ light beige/dark beige valcona leather with burr walnut inserts
My DIY jobs -
Replaced alarm horn,
Replaced Coolant Temp sensor,
Updated RNS-D firmware,
Installed Reversing Camera and Digital TV,
Dashcam Installation,
Retrofitted Rear Blinds,
Auto-Dimming Rear View Mirror,
Chrome Boot Struts
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2008 S8 D3 5.2 V10 - Engine: BSM, Gearbox:KLW, Colour:LY7F Suzuka Grey Pearl Effect, Interior:Black/Black with Silver/Black valcona leather with carbon atlas inserts, Bang & Olufsen, Heated windscreen, Soft-close doors, TV, DAB, AMI, Quad Zone, Auto Boot, Auto dim Mirrors, Keyless entry, PDC with rearview camera, AFS II Headlights
My DIY jobs -
Fan Jet Windscreen Washers,
Retrofitted Electric Side Blinds