Athul Jayson, the most probable culprit behind your issues is the middle metal dome sticker. You can see it is slightly misaligned, that most likely causes a constant contact like the button is always pressed. Lift the white sticker up with a blade and reposition it directly over the circle contact. See if that fixes it for ya.

Great guide! Came in real handy when I needed to replace a swollen battery.

Although the speaker removal part can be skipped completely.

Just remove the 4 Philip battery screws and peel off the black tapes securing the red/white speaker wires would be sufficient to allow the battery to slide right off.