@raiyan84355 make sure that you have your speaker turned on. You can check and adjust this via the menu by scrolling right > Settings > Sound & vibration. Next start your phone in safe mode to ensure it is not related to some third party software. To enter safe mode, turn the phone on and then long-press the volume down key. The word "safe mode" will appear in the lower left corner on the display. Test your speaker in Safe mode if it works then the issue is a bad third party App. You may have to remove those apps to get your speaker back. To exit safe mode, simply restart the phone.
If everything is properly set up and you still do not have a sound, it is possible that you just have a bad speaker. Since iFixit does not yet have a specific guide for the speaker replacement, use the one [https://www.nadiemellamagallina.com/en/manuales/smartphones-1/xiaomi-191/xiaomi-redmi-9-1289/altavoz-11489|from here] It was written is Spanish so you may need to use a browser that will do the translation for you, if needed. 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.
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