Why my mobile sound is very low even in high volume?

@shresthash56101 a lot of time this is caused by debris and grime in the speaker mesh. Use a soft brush and some isopropyl alcohol and clean the mesh, then re-evaluate.

If that absolutely does not make a difference, you can always replace the speaker. To work on your phone you can use the teardown from here 华为 Mate 20 X 拆解 or an actual guide from here It was written in Spanish so you may need to use a browser that can translate it for you. Replacement speakers are readily available at plenty of 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!