Simple and small free app that can always adjust the volume and is useful for movies music.

You can adjust the volume of music movies and apps and change from advanced options to phone call and speakerphone volume.

The green button on the notification bar is the hide function.
The speaker button indicates the volume and can be muted by double-clicking in the advanced options.
If you press and hold the speaker button you can activate the function that disappears in the advanced options.
You can resize the icon in the notification bar.

How to turn on/off button on screen with send intent in app like tasker:
1. Intent to hide the app
Action
com.oksoft.hide
Extra
hide:1
Package
com.alwaysvolumebutton
Class
com.alwaysvolumebutton.OnLock_BroadcastReceiver
Target
Broadcast Receiver

2. The intent to show the app.
Action
com.oksoft.hide
Extra
hide:2
Package
com.alwaysvolumebutton
Class
com.alwaysvolumebutton.OnLock_BroadcastReceiver
Target
Broadcast Receiver

For always visible rotate button put com.alwaysrotate instead of com.alwaysvolumebutton.
For other always visible apps just change the package name.