There are two cabin filters on the C6, both are located...
behind the glove box, on the left side. You have to drop the glove box to R&R the cabin filters. Although I haven't done this maintainence yet on my '06 A6, the procedure in the Bentley Manual seems fairly straight forward and relatively easy.
Found the process to replace the cabin filter straight forward. The glove box detaches after the removal of five 8mm screws. The link in this thread is helpful to locate the filters once the glove box is detached. Just reverse the process when replacement is complete.
Your A6 requires two identical filters to perform the task. I learned this the hard way and had to reorder another filter during the job.
As a non-smoker, I can not really tell if I improved my air quality. It just was just a task that I need to perform based upon the scheduled maintenance.
Hope this helps
__________________ 2006 Audi A6 3.2L
Retrofitted Navigation - Per forum, thanks Kevin & Pierreb - Successful
Added XM radio - Per forum, thanks Kevin - Successful
Updated MMI from version 1190 to 3360 - Per forum, thanks Pierreb & Ktwo - Successful
Updated MMI from version 3360 to 4140 - Successful