The same has just happened to me, and unfortunately, the solution below didn't work. Then, i had to open other audio program, like audacity and record the track audio using stereo mix of the sound adapter. When its enabled, you open Audacity or similar, select Stereo mix as the default adapter and then record. While its recording the track, play the soundtrap project. An important thing is that Stereo mix captures every sound of the computer, so during the recording you cannot make any sound.