Ok, this is embarrassing.
It turned out that (for some reason) the volume was set to -60 db in the playback audio setting, and this (for some reason) affects also the Kodi UI sound volume. The playback audio settings are separated from the general settings > system > audio, but the playback audio can be modified pressing the keyboard keys + and -. This, coupled with the weird behavior of Kodi that (for some reason) does not play UI sounds when using a controller, but only when using a keyboard.
It turned out to be a rather silly problem, but I leave it here hoping this can help others facing the same behavior.
There is some configuration that can be done by modifying the ALSA configuration file ($HOME/.asoundrc), but looks complicated and there's no 'one-size-fits-all' solution (depends on the devices present on the system).
You could try installing PulseAudio and configure the module-combine for simultaneous output.
Glad you got it fixed. You can try now to apply the overclock back, though I'd recommend you give it a try without it and see how the system performs. The Pi4 should be powerful enough for most systems.
@ramco That's weird, I never saw a TV/Remote who shuts down Kodi or the Raspberry, the power button is never assigned to o CEC button, it should only turn off the TV.
Probably it is your TV brand, never heard about that one. Each TV brand as it's own CEC implementation and name, yours might be one of the weakest.
You said you are running Kodi 18.2, Kodi is currently in version 18.8, update it and check, although I doubt it will fix your problem.
@Marco-G gpu_mem=320 is pretty much essential if you want to play high bitrate 4k hevc. You don't need as much if you don't play this kind of media.
About hdmi_ignore_cec_init=1, what it does is to not switch (the TV) to the Raspberry Pi/Retropie source when it is started or rebooted. It has no effect in anything else.