If you're using a receiver, most receivers include a HDMI Video out and in.
What I have done with my 5850 is connect a HDMI from the card to my receiver's HDMI IN, and then a HDMI from my receiver's HDMI OUT to my Monitor.
Unfortunately if you don't have a receiver, I don't think there's any other way to get sound to your loud speakers thorough HDMI.
Macs don't have mic-in (microphone input) jacks, they only have line-in jacks. Line-in jacks are "line level" audio input jacks, for receiving moderately amplified audio signals from another independently-powered piece of audio equipment like an audio CD player. Line-in ports expect power levels about an order of magnitude stronger than microphones usually provide.
Using the earpiece of a headphone as a makeshift microphone might kinda-sorta work, albeit at very low quality, on a mic-in jack, but it would never work on line-in. Even with very large over-the-ear headphones, the amount of power generated by your vocal sound waves vibrating the speaker cone to move the speaker coil through the magnets would be far too little to really register on a line-in jack.
Like jcomeau_ictx suggested, you can get a pre-amp for a microphone that amplifies the signal from mic-level to line-level. Or as NReilingh suggested, you can get a USB microphone that has it's own ADC (Analog to Digital Converter), so it basically acts as its own mic-in jack and sound input card.
If you have headphones that contain a built-in mic, you might be able to get that built-in mic to work on the headphone jack of your Mac mini, as long as the tip/ring/sleeve "pinout" of your headphones' 3.5mm phono plug matches the way Apple does the pinout for their iPhone headset with microphone.
(I feel like I've just consolidated and expounded upon what the other Answers have said, without adding much of my own, so feel free to vote them up instead of me.)
Best Answer
According to the Samsung spec page for your TV, the TV does have an optical digital audio connector. (It doesn't specify whether it's input or output, but it's probably labelled on your unit.)
The optical digital-audio input is what you want to use. You need a cable like these - optical TOSlink to optical TOSlink.
If not, check for analog stereo inputs (a pair of RCA-style inputs, one red, one white). You'd need a cable like these with a 1/8" stereo headphone plug to red & white RCA plugs on the other end. The mini plug goes into your Mac Mini's headphone output, and the RCA plugs go to your TV's analog stereo input. This will not be as high-quality as the optical.
Update: OK, it sounds like this "PC/DVI audio in" is another headphone-style jack like the analog audio output on your Mac Mini. In that case you just need a cable with the 1/8" stereo headphone plugs on both ends (some stores will label these 3.5mm plugs). The cable may look something like this:
Links to shopping sites in this post are for example purposes only. I am not endorsing any store or any product. Links are only provided to show what these cables will look like and how you can identify the correct cable in your store of choice.