The APCAST HDMI receiver IN and OUT the video and audio through the HDMI. If you connect APCAST to a monitor with built-in speakers, you will hear sound. But, in the car, because the speaker is connected separately, so, the audio must also be connected separately.
The AV cable has audio L and R wiring. please purchase a 3.5mm RCA Audio Cable, and connect it to the AV cable and AUX port.