serguntchik: Thank you for your reply! On my computer the path seems to be HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Crystal Dynamics\Legacy of Kain: Soul Reaver\1.00.000. However, modifying the resolution settings doesn't change anything :( The startup window still resets the resolution to 1440x900... I think gog should be ashamed to sell old games like this. I paid my money and all I get is the awful looping sound plus issues with the registry...
It looks like there are two places in the registry where the settings are (at least in my case but that's not GOG install).
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Crystal Dynamics\Legacy of Kain: Soul Reaver\1.00.000
and
HKEY_CURRENT_USER\Software\Classes\VirtualStore\MACHINE\SOFTWARE\Wow6432Node\Crystal Dynamics\Legacy of Kain: Soul Reaver\1.00.000
Only the second one seems to be taken into account so make sure that you checked that.
About selling the game in that state. In this particular case (sound skipping) unfortunately nothing can be done. This issue has been present ever since Windows XP and no fan or official patch could resolve this. So, as unfortunate as it is - it's as good as it gets right now.