Am aware of this bug but unsure how to replicate it. Might be due to monitor size of resolution. On Firefox it works for me. I hope you do enjoy the PC export builds.
In short slowdown effect was done using Engine.time_scale (This is because Kinematic2D uses delta for its calculations) but the Timer was divided by Engine.time_scale in order to give a time that is not affected by slowdown.
The music then had its pitch altered via script but you could also experiment with simply swapping the currently muted/unmuted sound if you want more control.