After 3 days, 100% success with no high pitch out of tune sound. I even switched off and on the hyptv box on purpose a few times to test it, to see if the high pitch sound would come back.

I am 99.999999999555551111111% sure what I have done has solved the problem. Ha.

NSTC was developed in America (electricity 60Hz) followed by PAL in Germany (electricity 50Hz) to overcome some short comings of the NTSC system.

From the web ...

"This is due to the fact that NTSC is generally used in countries with a utility frequency of 60 Hz and PAL in countries with 50 Hz, although there are many exceptions. Both PAL and NTSC have a higher frame rate than film which uses 24 frames per second. PAL has a closer frame rate to that of film, so most films are sped up 4% to play on PAL systems, shortening the runtime of the film and, without adjustment, slightly raising the pitch of the audio track."

Maybe clear your display setting (with none selected) and see if you will get the high pitch sound in a 3 days test?

Cheerio.

No need. Setting will be the same.

I have used both standby and power off modes ( I switch off the hyptv box every night). In both cases the setting stayed the same.

Strange, the setting is in HypTV and should not by itself reset because you are using a PC monitor.

best is change service provider. Digi infinite