It's completely untested (except for starting up) at the moment - it's too loud where I happen to be for me to do much in the way of other testing. It did happily start up, though, once I got past:
# pragma comment(linker, "\"/manifestdependency:type='win32' name='Microsoft.Windows.Common-Controls' version='126.96.36.199' processorArchitecture='X86' publicKeyToken='6595b64144ccf1df'\"")
EDIT: Added the source diffs. Most of the changes are to the build options (the diffs contain the migrated VS2012 project files as well) - apply with "cat patch | patch -p1" from the respective source directories. There's a patch included for both Audacity and wxWidgets, both working. The base tarballs can be found at the respective project websites.
wxWidgets: Don't let VS2012 build it automatically. As discovered by a few other people on some of the Win8 threads, VS2012 manages to start multiple processes that try to lock the same log file simultaneously and so the build breaks. I worked around this by building just the subprojects that I needed one at a time (right click on the project, build only [project].)
Audacity: Nothing major here except for disabling any and all inline assembler and letting it fall back on generic C/C++ defaults instead. The build doesn't end up copying the Nyquist files into the win\ARM\Unicode Release directory but since this seems like all it's doing is a straight copy operation, it shouldn't be a problem. Help isn't included (I haven't installed Python on any of my Win8 Pro installs yet, which is required to build the Audacity help), nor is it localized. Instructions for both are included in the Audacity source.
EDIT 2: Got a build of lame 3.99.5 for WinRT here:
Follow the instructions in the post to add MP3 export support to Audacity.
Full source code for Audacity and wxWidgets:
This is the full source used for building Audacity 2.0.2 and wxWidgets 2.8.12 for WinRT. The license is included in the ZIP file (audacity-src-2.0.2\LICENSE.txt - GPLv2; wxWidgets-2.8.12\docs\license.txt, wxWidgets-2.8.12\docs\lgpl.txt - LGPLv2 with additional rights). The Audacity build has been updated as well to include the license of both wxWidgets and Audacity.