Help with Mac Mini Audio Out to Dell Monitors and AC511 Sound Bar

I have two Mac Minis and I'm trying to get sound from my Dell monitors and attached AC511 Sound Bars..  One system is a 2010 Mac Mini running Snow Leopard and the other is a 2012 model running Sierra.  One monitor is a UP2516D and the other a U2515H with AC511s attached to both.  I know sound works on each because I've attached both to an Intel NUC through a mini-DP to HDMI cable and the A/V is perfect but I've been unable to do the same with the Minis.  So far I've tried mini-DP to DP and mini-DP to HDMI cables (both DP1.2 compliant) and neither provided audio.  I've also tried a HDMI to HDMI cable and it didn't work either.  The DP/HDMI connection and monitor are recognized in the audio and MIDI configurations but nothing produces sound, even the speaker configuration test in the MIDI setup.

Any suggestions?  Thanks for your help.