While working on bug 837430, I discovered the WAVE_FORMAT_EXTENSIBLE path was broken because wValidBitsPerSample was not initialized correctly.. Once it was, it was clobbered by later zero-initializations of wSamplesPerBlock and wReserved, which are part of the same union. With that fixed, WAVE_FORMAT_EXTENSIBLE works, so we might as well use it exclusively and remove a conditional. This also enables multichannel audio to work with WinMM, but more work needs to be done (in bug 521615) to map Vorbis channel layouts to Wave channel layouts.

Try: https://tbpl.mozilla.org/?tree=Try&rev=cc0958c2b20b