For iPhones, there is a background noise generator built into the phone (in Accessibility settings, controllable from the Control Center). On Android, a similar solution likely exists (one that continues to play even if other apps are playing audio).

You would just need to set one of those up, quiet enough not to be too annoying, but loud enough so that the stereo doesn't think its silent.

Also, there are audiobook players (for Android, at least) that skip over silent sections of the audio track.

The solution is an aux cable.

My LG OLED TV is connected via an AUX cable to a speaker, but still mutes its audio output after long-ish periods of silence. Cabled audio doesn’t help if the software sucks.

my TV now flashes "mute" every like 10 seconds in an animation... and also asks me to setup the TV every time with its cloud services...