Late 2012 Mini (the current one)
Audio in/optical audio in port
The Audio In port accommodates both digital optical input and analog audio input.
Analog audio line input is accepted through a 3.5mm mini phone jack. The sound input port accepts line-level stereo signals up to 24-bit stereo 44.1-192kHz sampling rate. It also accepts a stereo miniplug-to-RCA cable adapter for connecting stereo equipment to the computer.
Optical audio input is SPDIF format and uses a standard TOSLINK cable with a TOSLINK mini-plug adapter, accepting up to 24-bit stereo and 44.1-96kHz sampling rate.
Headphone out/optical audio out port The headphone/line out port accommodates digital optical audio output, analog audio output with a 24-bit, 44.1-192 kHz D/A converter, digital audio output up to 24-bit stereo and 44.1-192 kHz sampling rate and supporting encoded digital audio output (AC3 and DTS). For analog headphone / line output, use a standard audio cable with 3.5mm metal plug. For digital audio, you can use Apple iPhone headset with microphone or a standard TOSLINK cable with a TOSLINK mini-plug adapter
The rules of these boards prohibit speculation about future releases of any Apple product, but I think you will likely be disappointed in the graphics and/or audio if any present or future Mac mini. The mini has always been an intentionally low cost machine and features such as direct media interface with the video built into the chip are very likely to continue. If you are looking for high end video and audio the Mac Pro is definitely the computer you want, in the meantime you might take a hard look at the iMac.