Hi. I had the same issue when upgrading to the new OSMC version with Kodi 19. Done some research and came across this post:
https://forum.kodi.tv/showthread.php?tid=362663
This is regarding a Raspberry Pi 4 while I have 3B+, but the issue seems to be similar so I tried to add audio_pwm_mode=1 to the config-user.txt and rebooted.
And it seems to be working. I haven’t tested a lot of files, but at least the Youtube videos I know has issues before is now working. In the referring post, changing this mode to 1 results in a loud hissing sound, but I haven’t noticed that, so for me it seems to be working.