You replied to a 2 year old thread. Started a new one.
The U2515H does not control volume. The U2515H is simply a "pass-through" audio. The audio signal entering the U2515H via PC HDMI/DP/mDP out and exiting through its "Audio-Line out" port are controlled by the operating system volume settings and on the connected powered speakers. Assuming that you have adjusted volume within the operating system and used the volume knob on the speakers?