U2415, Amazon Fire Stick, AC511 USB Soundbar, no sound

I have tried to get a Fire Stick working on my Dell U2415 monitor. No problem with the HDMI video input.

The USB cable from the AC511 USB Soundbar is plugged into the USB downstream port on the back of the U2415.

I also have a jack cable from the Line-out on the U2415 to the line-in on the AC511 USB Soundbar.

I don't get any sound.

Does the USB upstream port have to be plugged into a PC/Laptop? Is it just power that is needed? Or a feed from the PC's operating system?