Speed-Dependent Volume: Function
WARNING: This page is about a different car, the 2006 Saab 9-5. However, it is still accessible from the selected car via links, so may be relevant.
The ESP control module transmits a speed signal on the P-bus. This is read by the MIU. The MIU then transmits the information on the I-bus, which the head unit can read to regulate volume. The level is increased when speed increases and is lowered when speed decreases.