When you have successfully loaded the project do you then Save it, or perhaps better, Save as.. using a new project name? Doing this should ensure that the correct soundpool file is now referenced in the project file.
The newest versions of Music Maker, IIRC MMM 2018 onwards, cannot use mxogg files - they use a different protection system which was changed in Music Maker - see this comment for a possible solution
Did you enter the serial number of the old version of MMM or did MMM 2020 recognise the installation of the old version in the Help, Actvated content dialog box if so then you may be able to download and re-install the Score Dark-Side.
After updating to the latest version of MMM did you download and install all your purchased content from the store if not I would advise you to do so - click the gear on the Store tab and select Download all purchased content