Not sure what model you own, etc. but my 2012 TTS does not have the Bose system and there is sound from the rear speakers but it sucks ass. Even after I had twin small subs and an amp installed in the rear connected to the stock HU, still hardly anything from the rear because the sub has to be connected to the front speaker setup I guess as its a component system or something. Like if I have my bass on and I fade all the way to the rear theres ZERO bass and the sound from the rear speakers (which should be the larger more powerful speakers I would think..) is shit. Its a shame because the car I just came from (bmw 135i) had its stock sound setup with nothing special but it sounded great compared to the audi's setup and I didnt even put a sub/amp in that car..Even suckier thing is in no way is it important enough to blow the 2-3 grand more for a full speaker/headunit swap out, the car is a lease and I just shouldnt need to spend that kind of money on the audio anyways..
Honestly I would say the audio is my main dislike about my TTS so far, I have a few other little things too that I miss from my bimmer (upper grab handles, automatic seat position memory, sunroof) but the audio thing kinda stinks, specially now that ive spent $1800 on adding the sub, amp and integrating the ipod into the stock hu (ipod integration works well atleast) and still the audio hasnt really improved much. I will say that any trance or electronic music sounds pretty damn good, but any kind of rock or anything like that just doesnt sound full or bassy enough unless i crank the volume wayy up past 3/4, and at that point its just killing my ears. The guys who did the install said that changing out the HU for a pioneer one or something that has its own full equalizer n shit would change it for the better perhaps, but honestly i cant see spending another 5-600+ dollars for something that may or may not even make much of a difference in the whole system, I shouldnt have too thats the point, really I think its just the audio setup as a whole in the TT's are just not great, period. I dont know bout the Bose system, some people say its wayyy better in every way, others say they dont see much difference at all. Who knows..