Mac Mini/Marantz NR1501 HT Receiver - No center channel

You may not realize this, but a 2.1 setup is two speakers and a sub, the .1 is always the sub. If you want to add a center channel speaker to a typical L and R speaker pair that's what would be referred to as a 3.0 setup. And then it's up to you to configure your AVR to send audio data to it...on my HK AVRs there's a Dolby surround mode called "3 Stereo," which takes two channel audio and "fills" a center channel, and is at least decent for music. It's best not to involve the Mini at all in this. Perhaps head over to the AVR forum, find the thread on your model, and ask folks if there is any trick to setting up a 3.0 speaker system with an HDMI input. (Now, whether you have any luck directing true center channel content, say from a DD 5.1 movie, to your center channel, is another thing...)

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.

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?