A free program for Android, by clownFace.
This VolumeSlider is a free application for Android devices that lets you control the volume of multiple audio streams at once. It can also change the brightness of those streams, so you can listen to all your music at the same time, even if the speakers or the headphones are connected to different devices.
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The way it works is that you can swipe your finger up and down the edge of the screen to control the volume of the current audio stream. You can also change the volume of all the streams on the same screen by sliding your finger from one edge of the screen to the other.
If you have multiple
Android
devices, you can set the application to work with all of them.