Why does my sound go all muffled

Penne Milloy  this could be as simple as a bad power supply (depleted battery) to a amplifier IC getting to hot (hence it takes time for this to open) and thus you get distorted sound. You may have to open it up, replace the battery and check the circuit board for and obvious damage etc., For the disassembly, check the excellent answer by Hacchan Kuroneko on this qurestion Why us my punk speaker battery drains easily

Post some pictures of your board with your QUESTION so that we can see what you see and hopefully can assist you further. iFixitでの質問に画像を追加する方法