Why can I hear the other side of a cassette?

The way a cassette works in the first place is that you have a big long strip of iron and when you apply a magnetic field to it, it gets magnetised. That's how you record things onto it. The varying magnetisation can then be converted into vibrations and you hear that as sound. The way that a double-sided cassette works is that when it's one way up it reads information from the bottom of the cassette and when you turn it over it reads it from the top of the cassette. So you have two strips: one at the top and one at the bottom with different information on it. So what's probably happening when you're hearing the muffled sounds is maybe the sensor that's reading the magnetic field is a bit too close to the middle and so you're picking up some of the other side of the tape, probably backwards.