I found more information about the Portal 2 turret choir song:It is called Cara Bel and it was (probably composed specifically for Portal 2.
I've been thinking more about the ending; For starters, let's assume that portal guns break the laws of physics and cost no energy when leaving a long-lasting portal open (likewise, no energy when objects pass through). The other option would be for some facility in Aperature Labs paying an energy cost for every object that passes through and exits higher than it entered, likewise for conversions of some angular momentum to other angular momentum.
What would happen if the portal to the moon were left open long-term?
The facility may run out of air. Would Terra itself be drained of air? I'm guessing not with one Portal, as our atmosphere has been slowly bleeding into space since it formed. There is a pressure difference; in the upper atmosphere, there's a lot less air to lose, but a very large surface area! One portal, about the size of a human, probably won't deplete our atmosphere, I think.
What if we were to instead imagine opening a portal to the heart of Jupiter or Sol? How do the pressures, heat, and toxicity of those atmospheres spewing onto a point on the surface of our planet figure? I am pretty sure the heart of Jupter would be less-bad, but I would not care to guess whether either would be locally bad or sterilise-the-planet bad.
Portals to other planets are an interesting thought experiment, and using them as such, against the backdrop of conservation of energy, is useful in helping us understand physics.