My point Dan is there is more than one way to set a default player in 10..the first that I listed was solely/specifically describing the way to achieve it within the players settings..of course this will differ from different players.
To achieve this within Windows is another way (Windows settings,nothing to do with PotPlayer settings)..
Just to recap the Windows settings:
Open the Settings app and then click on ‘System’. From the ‘System Settings’ section look for Default Apps option in the left side. Scroll down a bit if you do not find the option.
Here you can select the default app for Music files and Movies & TV. Click on Music Player and you will see a list of apps displayed, that can open these files. Select the one you want and click on it.
After you have done this, you will have successfully set the player of your choice as default media player in Windows 10.
If you wish to reset to the Microsoft recommended defaults, you can click on the Reset button towards the end.
I hope that clarified thing's...