The sound at low

@ydetxygdh could be a dirty speaker mesh or a bad speaker. Clean the opening for the speaker with a stiff nylon brush (something like a new toothbrush) and see if that works. If not, then it is most likely time to replace the speaker. Since iFixit does not yet have a guide for this, use something like this video At 2:18 you see how to remove the speaker. Replacement speakers are readily available at many online stores.

Now, if a new speaker does not resolve it, it is possible that the headphone jack is shorted out and that would mean you need to replace the charging board (daughter board) that has the headphone jack on it. Anything after that will be connected to the audio circuitry on your PCB and would require the schematics and the right tools to perform a boardlevel repair. Start with cleaning and by replacing the speaker.