The applies to official Libre Computer images! If you are using Armbian or other images, ALSA is not setup correctly and you need to set alsamixer fields.
If you are having analog audio problems or no sound from the CVBS jack on AML-S905X-CC, please check the voltage of your power supply. Due to Raspberry Pi 3’s poor power design, many companies made out-of-spec MicroUSB chargers that exceeded upper bound of the 5V±10%. We tested some power supplies that supplied 5.8V at 0 load. Raspberry Pi’s own power supply exceeds this safety margin and should not be used with any MicroUSB product besides Raspberry Pi’s own boards.
AML-S905X-CC’s low power consumption causes this issue to be very pronounced. The onboard DIO analog amplifier will not work when the voltage exceeds 5.5V and causes no sound issues. Please change your power supply to one that is compliant with USB-IF specifications.