Speed-dependent volume: Function
WARNING: This page is about a different car, the 2009 Saab 9-3. 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 that is read by MIU. MIU transmits information on the I-bus that the head unit can read to regulate volume. The level is increased when speed increases and decreased when speed decreases.