The sound may have been recorded, yet the final audio product depends on how PD has recorded and subsequently processed the sample. Further, the DB7 should have the most subtle of the exhaust notes (2000 model before optional Vanquish-like sports system) yet it is a great deal heartier than the DB9. In fact the DB9 comes across as being almost unrefined in comparison with the silken and complex sound of the DB7/Vanqusih.
Once again, it is my view that PD has attempted to create differences in sound between the vehicles. In this case I would suggest that it is knob twiddling gone wrong. (innuendo not wholly intended)
For those who havent seen it, here is an image of PD with DB9 at the Nürburgring.