I think Microsoft should open source them.
No real need to, as there are already numerous choices that are open source as well as cross-platform. There is SDL for graphics and audio, plus PortAudio; both of these are open source, with open source C# wrappers available. For games there is Unity3D, among others; there is really no need for them to do so. The continued requests aren’t going to accomplish anything, since they’ve pretty much made the decision to not open source either one.
MonoGame is an open source implementation of the Microsoft XNA 4.x Framework.