The AVC-LAN is part of the Body Electrical System Control and manages all audio and video related functions. The audio bus is described in Toyota's own words as follows:
AVC-LAN consist of audio visual systems such as the multi-display, navigation ECU, radio and player, stereo component amplifier and gateway ECU. Gateway ECU has communication circuit to correspond with different types of communication data. Different types of communication data can be shared among communication parts after it goes through gateway ECU (System Circuits: AVC-LAN Bus, “Prius Wiring Diagram” 80).
The system itself can be considered a subset of the IEBus Standard, which was delevoped by NEC electronics for automotive use. It is slightly faster than the BEAN bus and has more data per command, but is still much slower than CAN, since realtime speeds are not required.
(Jeremiah J. Flerchinger Source)