Audio Crank Clamp

Lock the minimum and maximum audio volumes on your device. You can lock each type of audio volume separately. It helps prevent accidental or unwanted volume changes. There is an password option to lock launching the app. It can run on start up (optional), has a home screen widget, and has a dark/light/black/amber theme option.

Try the Lite Version
https://play.google.com/store/apps/details?id=com.grantojanen.audioCrankClampLite

More Details
Offline: Yes
Lightweight: less than 1MB
Multi-Window Support: Yes
Descriptor: audio volume locker

Permissions Info
run at startup: This is used to restart the volume lock (if it's enabled) when device is rebooted. This can be disabled in the settings.
Foreground Services: This is needed to run the volume lock service on Android 9+.
modify Do Not Disturb settings: This is needed to set a minimum volume lock on the ringer, notifications, and alarms on Android 6+.