Why my speaker is not working good

Because your speaker has faced internal damage, try repairing it by going to a repair shop.

@soumyajitsaha not sure why you would think that the speaker was water damaged. Let's hope it is just the speaker. There is really not much you can do other than to replace it. Since iFixit does not yet have a guide for this, use something like this video You will find out this phone is a bit of a pain in the rear to disassemble. Replacement speakers are 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 a device with the same issues as you do. 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!