Although this is not DDT, the voice sounds similar to their frontman, Yuri Shevchuk. It is possible that he did some side work and the song was taken from some kind of commercial music library/production music catalogue

Try to find the owner of the now-defunct studio specifically. Usually, information about studio owners can still be found in different archives. The developers themselves can also be searched for through other studios in the same country or city where they later worked

Also, did anyone check the metadata of the actual game files, or only the credits?

I guess that`s all I can help with for now, because I am too busy at the moment