One detail I loved was the change in the music when you enter and exit the mirror world. Very nice little immersive touch.
Viewing post in Flip Flop jam comments
The type of filter itself is one of many that are included in the gamedev software that I use (Construct 3). Once the condition if fulfilled, the filter is applied.
My friend -- he's an amateur musician and wrote the music for this game -- told me that what I'm looking for is called a low-pass filter, and also gave me a few pointers on how to configure the parameters (frequency, detune, etc.). I fiddled about with it until I got a satisfactory result.