Yeah, I don't think I would have consciously noticed it if I hadn't just implemented something similar for a slow motion effect - but it makes such a big difference. I'd love to know how you achieved the sound technically. How did you filter it? In engine or did you preprocess it?
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.