A volume slider

Build a slider that sets the Windows master volume and follows it when the volume changes elsewhere.

You need: a paired device (see Pair a device).

1. In the editor's Toolbox, click Slider. Resize it so it is wide (a full row works well).
2. In the Properties panel set Min to 0, Max to 100 and Step to 1. Give it a caption such as Volume.
3. Under Variable driving its position pick system.audio.master. The slider now shows the current volume and moves when it changes.
4. Under Actions, on Value changed, click + Add action and choose Master volume. It applies the slider's value.
5. Click Save.

Drag the slider on your phone: the Windows volume follows when you release. Change the volume with your keyboard's volume keys and the slider moves.

Add a mute button

1. Add a Toggle with the text Mute.
2. On Turns on add Mute/unmute with Mute. On Turns off add the same action with Unmute.
3. To keep the toggle honest when you mute from elsewhere, use a dynamic rule on its background: if system.audio.muted equal true, red.

TIP

Instead of a toggle you can use a plain Button with the Toggle mute action, and a dynamic text such as {system.audio.muted|MUTED/LIVE}.