Got a brand new Mac mini - plugged it in and played with it. Then I realized that it had no sound. Even the inner speaker was grayed out. Adding an external speaker did not help. And a quick google did not help.

I went to the Apple Mac mini support forums Apple.com - Support - Discussions - Mac mini and I found a plausible theory in Not getting any sound -.

CDW had opened the mini to upgrade the memory for me. Once I learned how to open it up, I did so and found this ribbon cable loose:

Upon connecting it:

I ran a quick test with the cover off and discovered I had fixed the problem. Also, note that taking the cover on and off gets easier with practice.