Speech is certainly degraded when wearing a mask. I hadn’t realized how much I depended on reading lips to fill in gaps in my hearing until people started wearing masks. Without visual cues I struggle in conversations. To understand what is said I now turn my radio and tv up until it is painful to others. I must now get very close to someone who speaks in a normal voice or I understand virtually nothing they say. Not a good thing to be doing in this time of covid.
As far as what happens acoustically my guess is the mask becomes an attenuator as well as shifts the spectrum to lower frequencies. The acoustic response depends on the material of the mask. Heavu cotton will muffle sound. The lighter blue synthetic material seems to affect speech the least. Less mass means sound waves pass through with less attenuation? In general the mass of the mask acts as a damper, lowering the volume and pushing frequency content lower. Think of it as less air flow through the mask. All such conjecture is anecdotal of course. I suspect one could find more info in JASA and other such publications.