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The bottom speaker of my S9 plus isn’t working. I bought this S9 plus broken from another person and fixed it by buying a new screen replacement and moving the inner parts onto it. I was unable to check if the speakers worked or not since I bought the phone with both screens cracked and a dead battery. The top speaker works fine but the bottom speaker doesn’t work at all. I’ve tested the speakers in the secret menu (the #0# menu) and no sound comes out of the bottom speaker so I know it’s not a glitch. Which part is it that I have to replace?

They sell replacement speakers for the s9/so plus. You can get them from multiple suppliers. Ifixit should have a instructional video on how to change it and YouTube should have many more videos.