Pi4 has no audio on second HDMI port
The Raspberry Pi 4 has two micro-HDMI connectors, HDMI0 is the one closest to the USB-C power port, and HDMI1 is the one closest to the USB 2,3 connectors. By default, the main HDMI port is HDMI0, and the audio is directed to this port.
According to this post on the Raspberry Pi forum, the Raspberry Pi cannot send audio to both ports simultaneously, but you can select which port will get the audio with a line in the /boot/config.txt configuration file.
To set audio output and CEC on HDMI1, add this line in /boot/config.txt: "hdmi_preferred=1" and reboot.