Hi Abhilash_C_S,
Thank you for using Apple Support Communities. I understand you're having issues where your notifications are causing all sounds to be played at a lower level. I'd be happy to assist.
Before we continue further do you have a backup? If not that's alright. It's always recommended to have a backup before troubleshooting potential issues. If you'd like to make a backup here is a great resource: How to back up your iPhone, iPad, and iPod touch
There are two things I'd like you to try. The first is disabling Attention Aware. You can do so by navigating to Settings > Face ID & Passcode and turn off Attention Aware Features. Here is an article on Attention Aware and what it does: About Attention Aware features on your iPhone X or iPad Pro
If your issues persist after that I would then recommend resetting your settings. Here is a great resource on how to do that: Return iPhone settings to their defaults
Thanks and take care.