The speaker output became poor

@mesfintigist your description is not very specific. Everybody may have a slightly different opinion what poor output actually is. It is possible that you have some debris is the speaker mesh and cleaning it with a small brush, something like a toothbrush could help.

It is also possible that this is a speaker that simply wore out and may need replacing. Since iFixit does not yet have a specific guide for this, use the one from here It is written in Spanish so use a browser that does the translating for you, if needed. Replacement speakers a readily available at many online stores.

While you work on your phone, take lots of pictures. Once you are done with it, take those pictures and create a guide for iFixit. That will help the next person that has the same issues with their phone. It's easy and fun to create these guides. https://ifixit.com/Guide/new The iFixit community will appreciate this.

Repair is War on Entropy!