DP++ to DVI has no audio signal to pass to HDMI. There are no pins on DVI defined as carrying audio signals.
DP++ directly to HDMI can have audio.
DP++ >>>>> DVI >>>>HDMI does not have a path for audio.
You may have an internal speaker that audio is going thru but this is not related in any way to an external monitor. The Adapter below is DP++ directly to HDMI and would have audio EXCEPT if the port its plugged into is not ++ then it still wont work. I do not see the ++ logo on the 790 case nor do I see the audio codec mentioned in the specifications.