Hi Hias! Believe it or not, I found the problem. You can see it in the attached picture.
It's beyond my understanding, it didn't crossed my mind that this wireless keyboard could interfere with another USB device and cause such a problem. I used it in the last two years on my Rpi3 without any problems and when I bought the Rpi4 I've just hooked it in an USB port and left it there.
A few minutes ago I said to try one more time the RPi4 from the ground, without HifiBerry Digi+ Pro attached and I also removed the keyboard dongle. I flashed Volumio in first place and when I tried to play a 96/24 FLAC the big surprise came up. It plays everything as it should on Volumio.
I flashed LE 9.2.6, it's working perfect, no problems at all.
I did a few tests and I tried all four RPi's USB ports with different combinations. Everytime I was inserting the keyboard dongle in the RPi the USB audio output was messed up.
I don't know why this is happening and if this is normal but is good to know about this issue. Maybe it could be solved if it's related to the software.
Thanks a lot for your effort in helping me and I'm really sorry for the time you lost but, as I said, it didn't crossed my mind.