I am thinking about buying the Pi Zero kit here.
Would it work with a UE Boom 2? It has a 3.5mm jack plug and bluetooth connectivity.
You could use the bluetooth function like this:
Or, if you wanted to use the audio jack, you could route the audio out through the GPIO and connect it to a jack plug to bare ends wire (like this). You would hook up the audio jack's right channel to GPIO12 and it's left channel to GPIO13, and it's ground to any of the Pi's Gnd pins. You then run
pigs m 12 0 && pigs m 13 0 (Full instructions are here, and the pinout of the header on the audio jack cable is on the website).