Muffled audio has four common causes, and only three of them are fixable at home.
Narrowing down the cause
A blockage, a software setting and a failed driver all sound different. Identifying which one you have decides whether this is a two-minute fix or a repair.
What to rule out first
Check that no Bluetooth device is connected, that no case is covering the grille, and that volume limits or accessibility audio settings are not active.
Try the free speaker cleaning tool and measure the result before and after.