Speaker sound decrease automatic

@peethaganesh could be an issue with the volume flex or with the software.

Try to start your phone in safe mode and see if it works properly in that mode. If it does, you know the issue is actually a 3rd party App that causes this. You will have to uninstall those Apps. Here is how to place it in safe mode.

After that, I would start with a new Power Button & Volume Button Flex Cable. Take a look at something like this video since iFixit does not yet have a guide for that. The flex is readily available at many online stores.

If that still does not resolve your phones issues, it may be an issue with the main circuit board.