I unticked 'release driver when app is in background,’ and in Cubase 10 Pro THAT is working for me so far. My evidence dies that I am typing this and listening to Cubase at the same time.
BIG LOTS OF THANKS!!!!!
This was driving me crazy.
It's likely that the final audio file was not just a raw "addition" of the original files, and it was 'mixed' using various techniques (EQ,compression etc.) Therefore it's very unlikely you will be able to reconstruct the exact same mix unless you are willing to spend a lot of time with your ears.
There's no particular software that will do this, other than ...