I understand that your iPhone's volume is decreasing or muting itself. I suggest taking a look at the information below, as there may be an issue with the volume down button on your iPhone.
What to do if a button or switch isn't working
- Restart your device. If you can't restart your device, try to force restart it.
- Remove any protective films and cases that might block the button or switch.
- Check for debris or dirt around the button or switch, and clean with a soft cloth.
If you need help with Touch ID, learn what to do.
If the button or switch still isn't working, is damaged, or works intermittently, you might need to set up service for your device. Contact Apple Support, then follow the steps to complete your service request.
Alternatively, if you're using headphones or a bluetooth device when this behavior occurs, you may need to check to see if this occurs without the accessory connected, as the connected device may have an issue with its volume buttons.
Let us know if this helps!