An audio and video transmission system includes a multimedia device. The multimedia device includes a high-definition multimedia interface (HDMI) receiver, a first transfer circuit, and a first universal serial bus type C (USB-C) interface. The first transfer circuit is configured to transfer a first audio signal output by an audio channel pin of the HDMI receiver into a second audio signal in a universal serial bus (USB) interface format. The first USB-C interface is configured to transmit the second audio signal. The HDMI audio channel pin is an audio return channel (ARC) pin or an enhanced ARC pin.