"How well do they stack up?" All depends on the requirements of what you are trying to do. Are you looking for adequate voice over limited bandwidth transports (think 2 way radios) or are you looking for highest fidelity lossless music reproduction? Opposite ends of the spectrum. If you want a broad statement, then yes you can probably find an opensource codec that perform about as good as a closed source one. Lots of the closed source ones put out reference implementation that can be or is the basis of open source ones, but those may have very restrictive licenses.
Android: based on Linux, no? So you could argue it's more likely to support open source tools. WMP and QT? Speculation, but supporting other formats gives customers a reason to stay on Windows and OsX
All I was trying to point out that Amazon (and other providers) is driven by customer demands. A large enough group of customers demanding "release in format XYZ" will get attention, while one or two people asking questions elsewhere won't get Amazons attention.
My opinions only; I don't have an Amazon account so they sure as heck won't listen to me.