The audio is coming from the mini so its involved.
Sorry, of course it is involved, my point is don't try changing anything on the Mini side just yet, don't give in to the temptation to go into "Audio MIDI Setup" just yet and try to configure speakers in there
. That's how many folks mess things up. You really have to know what you're doing in there, so don't make that newbie mistake, do not try to "configure speakers." There have been many years of threads about x Mac not working with y receiver. Almost all of them come down to the user not knowing how to configure his AVR properly.
I already enabled the center channel on the receiver.
You may think you have, but there may be more to it than what you've done.
What I'd do is start testing with music from just one app, iTunes, though close it at this point. Then open Audio MIDI Setup in your Utilities folder in Applications, click on the HDMI output tab and set it to 2 channel 16 bit and 44.100 kHz. Then open iTunes and start playing some music. If your AVR and center speaker is properly enabled you should "hear" sound coming from that center speaker. Do you?