can you add speakers to a Mac mini?

The Mac mini has a single small built-in speaker, like den.thed mentioned you can plugin external stereo (2.0) or stereo plus sub woofer (2.1) speakers and like what woodmeister50 said these need to be powered speakers.

It used to be on older Mac mini models you could use a mini TOS-Link optical connection to connect an AV Receiver but newer Mac mini models including the current one do not support this. What you can do is use the HDMI connection to still connect to an AV Receiver and run the audio over the HDMI connection and then the AV Receiver connects to the speakers.

A USB external DAC - Digital Analogue Converter can also be used to link to external speakers and some of these also support 5.1 surround sound speaker setups. These are however more commonly used by professionals.

Yes you can.

Besides headphones, you can plug most any 2.0 or 2.1 speaks system into the Mac Mini's 3.5mm headphone jack.

You can also use, most any kind of wireless Bluetooth speakers with the Mac Mini and other Mac's.

To a lesser degree and more rare, you can also plug and use some USB speaker systems.

Note that any speakers that you connect to the Mini need to be self powered.

The headphone output does not have sufficient power to drive speakers.