There you go.  The monitor would not power the sound bar via usb if the monitor is not first receiving dc power from a pc via a USB upstream cable.  All the USB ports of monitors work like that.  The ac power of monitor only supplies power to the lcd screen.
When I connect the Dell Soundbar SB521A to my laptop, it automatically turns on. However, when I connect the Soundbar to my P3223DE, the power light flashes once but does not turn on? According to the data sheet, it is compatible. The P3223DE is not connected to a PC. I'm using it to stream Roku. Roku works fine, just no sound.
P3223DE, Soundbar SB521A won't power on