Maybe the grills are muddied up with dirt?
Occasionally I take a little pin/needle and very carefully remove lint and such from the headphone port, lightning port, and less commonly, the speaker grill. Extra emphasis on very careful. You'd be surprised how much crap goes up there.
With the grills though you have to be careful with your cleaning method. You want to make sure you don't accidentally force dirt into the holes as opposed to out of the grill area.