Correction: it's a function of the signal frequency and bit depth (or noise level, whichever is limiting). Maybe I should update the article to make that clearer.
Galinstan is a less toxic alternative to mercury that is liquid at room temperature (and a bit lower). It's an alloy of gallium, indium, and some other stuff.
BubbleUPnP will talk to a stock FireTV using some other protocol, no sideloading necessary. There are presumable other apps that do the same, but that's the one I've used.