Mac Mini - no sound trough DISPLAYPORT after waking from sleep

Hi!

Since today I have the problem that there will be no sound output when waking uo from standby mode. Only Rebooting helps or changing the sound output to headphone. There is no error message, the display port sound output is listed in system audio preferences. All the workarounds like deleting the Audio config files or reseting the PRAM did not help.

Analog Audio output works fine all the time.

I have a Mac Mini Mid 2011 (Yosemite 10.10.3) attached to a 34" Display from HP (Envy 32) via DisplayPort.

This Problem does not occur when I attach a Macbook Air via DisplayPort or connect the Envy32 via HDMI.

Any idea what to do ?

Gospo