Stereo analog or digital audio input combiner

A friend of mine has this question:

"Can you send me some links to any device((sic)arudiono, zigbee, etc) that can take two analog or digital audio signals, combine them, then output it out as one sound source. Basically like a mixer but i'd like it as small as possible."

How can one do this?

Op amp Summing amplifier. Basically each audio source thru a resistor to an opamp input.

http://www.electronics-tutorials.ws/opamp/opamp_4.html

If digital do you have any idea which digital standard? Analog is much easier but doesn't really need an Arduino or other microcontroller.