Took me a moment to see if what you were asking was what you were asking.
Just in case, remember that MIDI connections are basically data only; they are used to interface with keyboards/controllers and sound modules only, and audio does not pass through them.
I pulled up a page on the Mia Midi.
It seems that you have two 1/4" jacks in, and two 1/4" jacks out, plus two RCAs on the dongle for SPDIF in/out.
I'm going to take a wildy accurate guess that the 1/4" jacks are not stereo, so not only does it mean you can't record 4 tracks through both, you can't just plug headphones into one of the outputs either.
If you can get a Y-adapter that can get left out of one 1/4" out, and right out of the other one, you may be able to drive headphones off of it. However, the output volume may not be to your liking.
The best option is to get a reciever or headphone amp and connect the two 1/4" outs of the soundcard to the receiver or amp and listen through that.
I would also advise that if you wanted to record 4 tracks simultaneously, this is probably not the card for you, as SPDIF to analog converters can be costly.