[Solved] Mono audio output with HDMI TV speakers on Raspberry Pi 3 (RPi)

I find that downmixing stereo to mono makes it easier to hear speech with less intrusive loud music and explosions, it seems to have a center boost effect as most unwanted sounds are mixed in stereo to give them separation. This can be done on Raspberry Pi 3 with LibreELEC 10.0.3 connected to a stereo TV via HDMI by creating the following ALSA configuration file:

/storage/.config/asound.conf

pcm.monodownmix {
    type route
    slave.pcm "hdmi:vc4hdmi"
    slave.channels 2
    hint.description "Two-speaker mono"
    ttable {
        0.0 0.5 # in-channel 0, out-channel 0, 50% volume
        1.0 0.5 # in-channel 1, out-channel 0, 50% volume
        0.1 0.5
        1.1 0.5
    }
}

Reboot and go to Settings > System > Audio > Audio output device and select ALSA: vc4-hdmi (monodownmix), MAI PCM i2s-hifi-0. The front left and right channels are combined 50% and played through both speakers. Note that audio clipping may result if 100% of both channels were to be combined. The trickiest part for me was getting the right device name for slave.pcm. The device does not appear in aplay -L unless it has a hint.description.