The Audi Sound System (option code PR-9VD) in the A8 D5 Facelift includes a dedicated subwoofer that reinforces the low frequencies of the 10-speaker system. However, in the factory MMI Sound menu, only Bass and Treble sliders are exposed. The subwoofer level itself is fixed at a default value and cannot be adjusted independently.
This activation unlocks a dedicated Subwoofer slider next to Bass in the MMI Sound settings, giving you independent control over subwoofer level — critical in a flagship cabin engineered for silence, where every fluctuation in low-end response is obvious.