@viratkumar most common issue is a clogged speaker mesh or the speaker opening. You do want to clean this with some isopropyl alcohol and cotton swabs. If that does not resolve it, you will have to open your phone and inspect the speaker, connections etc and eventually replace it. Here is [https://www.sosav.com/guides/mobiles/oppo/oppo-a5-2020/external-speaker/|the guide] for the speaker.
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 problem 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!