8P based cars, namely A3, S3 & RS3 with Bose installed have the audiopilot system, which basically monitors the internal sounds in the car & changes the output of the stereo accordingly.
On non Bose cars, this uses the gala system, which monitors the cars speed & depending on the menu settings of 1-5 or off, will change the volume based on this setting.
The problems faced with some Bose facelift cars, namely 09 onwards are, there is a hissing when using making a phone call, if fitted &/or the volume erratically goes up/down all by itself when using head unit, even with the engine of & no distinct difference in background sounds, that would cause the audiopilot to make changes, this by itself is not correct & apart from being extremely annoying, is infact a fault in my eyes.
There have been varied discussions on this, but there are between 1-3 things that need to be employed to correct this issue.
1: There is a known Audi TPI 2029158/5 specifically calling for the replacement of the Bose/Audiopilot microphone to try & resolve the issue, for some this doesn't completely eradicate the issue & possibly requires the next 2 steps to fully resolve.
2: Adjust the wiring within the amplifier loom to disable the audiopilot system, a bridge of 2 wires has to be made between wires 6 & 24.
3: Make some changes to the coding to enable gala.
I have personally done this a few times & it seems to have helped, there has been a question if the gala system works afterwards, but for most they are just more happier to leave as is & not have the volume going up/down like a yoyo to extreme's.
If your rns has been retrofitted, the dealer may question if its under the TPI, but push it & they should change, if not then we can do the above for you, albeit at a cost, but that's your choice