[visiondiez]

I'm a bit confused about the audio out jack on the rear of the mi box. I read that it's a hybrid solution with two ports, but I'm no sure how it works. Do I need to purchase any specific adapter?

I don't have this box, but this is a pretty common type of jack on compact laptopts and audio equipment. From Wikipedia:

https://en.wikipedia.org/wiki/TOSLINK

Mini-TOSLINK

Mini-TOSLINK is a standardized optical fiber connector smaller than the standard square TOSLINK connector commonly used in larger consumer audio equipment. The plug is almost the same size and shape as the ubiquitous 3.5 mm stereo minijack. Adapters are available to connect a full-size TOSLINK plug to a mini-TOSLINK socket. Combined 3.5 mm jack and mini-TOSLINK sockets exist which can accept a 3.5 mm jack or a mini-TOSLINK plug; mini-TOSLINK plugs are made 0.5 mm longer than electrical jack plugs so that the latter are too short to touch and damage the LED of combined connectors. Many laptop computer and portable digital audio equipment models, such as the Google Chromecast Audio device and Apple AirPort Express and iPod Hi-Fi, use these connectors that allow for the insertion of 3.5 mm analog (electrical) headphone output or microphone input or mini-TOSLINK digital (optical) output.

Your Y cable will work without any problem. The jack is designed for that.