What is the right device name format for volume-widget?

[TD-Sky]

Today I find that my volume adjustment button doesn't work cause the device parameter of volume-widget.
The "pause" or "default" value has no effect.
I also use a variety of tools to search the name of available device. But there are too many property maybe the device name for control. I want to know the exact name format volume-widget needs.

[TD-Sky]

I compare the current version with the past version that volume-widget worked and find that only set device = "" then it works.