The human ear perceives volume in a logarithmic way, that’s why the dB-Scale is a logarithmic scale.
There’s also no difference soundwise because the result of summing up this 2 sine waves is again a sine wave with the same frequency. If you use 2 sine waves with frequencies that are an integer multiply apart to each other, let’s say 200 Hz and 400 Hz, then mofifying the phase of one of these sine waves results in very subtle changes in the sound perceived by the human ear.
The human ear mostly distinct different sounds by different overtones. You have to do a frequency analysis of a sound to get it’s characteristic.
My ears percieve a rising volume from 0:33 to 0:50. Maybe your audio system doesn’t work as ecpected?