I tried making a new project file, then added the music (.mp3), which worked just fine by itself. Then I added the commentary track (.wav), seemed alright. Finally adding the ambient sound (also .wav) and playing them all together worked just fine the first time around.
Adding them one by one, seems to work for a limited time before they somehow “merge” (?) I’m honestly baffled and having a hard time explaining myself.
The problem just appears out of nowhere it seems. Could it be that Vegas isn’t fond of working with both mp3 and wav formats at the same time?