How to connect Mac w/ DVI-only to HDMI-receiver ?

You could use the Gefen DVI Audio to HDMI Adapter, but considering its price ($300), buying an AV Receiver which allows the mixture of TOSLink Audio + DVI/HDMI Video inputs into a HDMI Video+Audio output may be a better future-proof investment...
Or reselling your current Mac Mini and buying a 2011 model, which does support audio+video on HDMI and has much better graphics performance.

Note that to get the TOSLink optical output, you will need a (cheap) mini-jack to TOSLink adapter.

geeji's point is, you should simply be able to send video only into an HDMI input, i.e. DVI into HDMI, and then assign an optical audio input to that, and be fine. Most good AVRs allow this with no need for any special adapter.

But, the 2005 PPC Minis didn't have optical audio built-in, it wasn't until the Intel Minis were released in 2006 that they got digital audio out built-in. Frankly, it's not worth spending any money getting a PPC Mini hooked up to an AVR, nor the first Intel Minis hooked up to an AVR, unless you were only interested in music. And in that case a 2006 Mini would perform well, so if you are getting that Monoprice adapter to work it may be you have an Intel Mini after all. If you're at all interested in video, though, and high def at some point, you need something newer. Those first Intel Minis were very under-powered when they were released, and are very poor performers 6 years later (unless you upgrade them, which many folks have done.)

chefklc expressed it much better than I did, I was not familiar with the Pioneer VSX-1121, but if I understand correctly the audio priority table page 16 of its User Manual, and the note about DVI+sound page 78, you don't need any external adapter if you make the proper audio input assignment and use a DVI->HDMI cable for video (exactly as I do from my Mac Mini 2009 to my Yamaha RX-V3800).
And if you have a 2005 G4 Mac Mini (instead of a 2006+ Intel one), the mini-jack output will work too, except that it will give you analog stereo instead of multi-channel 5.1 PCM TOSlink optical output; in both cases your AVR should also be able to merge into a HDMI output.