The speakers are of course 100% done.
I replaced the speakers on 98 and 02 A6s. The speakers had holes and were literally like tissue paper. I really surprised any sound was coming out of them.
Check for sub wolfer in the trunk, it might not have one or would be hidden in trunk somewhere, maybe behind the liner panel near the passenger side rear wheel. Just to see if there is one and is it bad also.
I would simply replace everything. Maybe do it in steps.
Is the radio unit frame double DIN? If so then you have a lot of good options.
You most likely can do the speakers yourself. Find out how to remove the door panels. Remove the passenger front and rear panel. You should be able to manually open, close , lock and unlock the door from the inside. But no power locks or windows.
Then you can for see what size speakers you need and not guess or rely on someone else's best guess. Put the door panels in a secure location. I would NOT keep them in the car as they will bounce around and break/get scuffed up, or put them somewhere they can get knocked over and stepped on like hallway, working garage, etc.
You then get the speakers you want online or at a store. Install the passenger side ones and test. Keep the volume medium to low since you still have the other old speakers. Then you should be able to more quickly install the drivers side ones.
I put Alpine SPS 610s on my A6 and they sound good to me.
Any basic speaker will for sure be an upgrade and you check local stores or best buy. Just my opinion, not worth spending much over
Take the time to clean out the interior of the door panel and spay some silicon lube, preferred over WD40 on the moving parts. Check any window regulator clips for being loose or cracked, You may want to replace if they look damaged. Again door panels may be off for a week or two.
Try to use butt connectors for the wires rather than twist and electrical tape. They will hold up better.
Carefully remove vapor barrier that is FOR SURE needed, do not just throw it away. If it is rotten, really damaged, then you can patch with HVAC foil tape. Duct tape will not work. Or make a new one from heavy plastic. If mastic putty is dried out, I have had to scrape that off and use HVAC foil tape to seal vapor barrier to door. I am doing this for my cars, like and old Blazer and my old A6s, so there may be a better idea or a pro may get more of the black mastic putty. Not sure on that point.
You will need a D battery to do a polarity test. I would simply test them and not waste time trying to look up wiring diagrams. With four pieces of masking tape, put one 3 inches from where the wire is connected to the speaker. On the same wire another inch up . Mark with a sharpie A.on both pieces of tape. Do same for other wire mark B. Cut wire between to pieces of tape. Put battery to speaker, mark polarity. When you get + to + to speaker will move up. Then mark the tape of the wires in door panel with the + or -. Now you know which wire is which for the new speaker. I would leave the tape on their for any future reference.
And you may need a solder gun. They are not expensive.