I have the Dell AC511 soundbar connected to my DEll U2715H monitor.
When I change from DVI input to HDMI Input, the sound dissappears. Or, to be correct the sound remains with my DVI unit even after switching to the HDMI input.
I guess this is a design bug with the monitor as I did not have this problem with my previous monitor, the U2415H.
Try the following to see if that is the issue.
Change the Playback Device configuration in Control Panel
Go into the audio playback devices settings (Start > Control Panel > Sound) and change the default audio device