This is a bug that was introduced into 4.6. In the song page, On my iMac running Mohave, I'd lower the master channel fader significantly just to notice the change in the Project Page. After updating the song mastering file, the waveform in the Project Page looks correct. However, the audio is not correct. I also tried this on a brand new PC running Windows 10 Pro. Same problem. After reverting back to 4.5.5 on both machines, the Project Page plays the audio correctly. I discovered the problem by mistake because anytime I had to go back to the Song Page to make any corrections, they were subtle. So, I did not notice the changes were not occurring in the Mastering Page. Reading through the user forums, it appears 4.6 is full of bugs. This is just another one of them. So far, I'm sticking with the most stable release (4.5.5).