Hi dulanjan48,
Thank you for posting in Apple Support Communities! Does this happen on any other app? Are you able to hear your contacts if you are making audio or video calls using WhatsApp?
You mentioned this is happening on iOS 15.4, iOS 15.4.1 was recently released. You can go to Settings > General > Software Update to install it.
Update your iPhone, iPad, or iPod touch
Be sure to back up you data before updating. If you use iCloud to back up, you can do this by going to Settings > Your Name > iCloud > iCloud Backup, and tap Back up Now.
How to back up your iPhone, iPad, and iPod touch
Once up to date, you can confirm this continues. If it only happens with WhatsApp, you may want to reach out to the App Developer. You can go to the App Store, search for WhatsApp and tap it to open up the product page. Scroll to the Rating & Reviews section and tap App Support.
How to contact an app developer
Have a good one!