WaveShop might fit your constraints. It's free and open-source, it's less complicated than Audacity but still has plenty of editing features, and it has a straightforward, uncluttered native Windows interface. It's also thoroughly documented, has 64-bit and portable versions, has good surround support, and is being actively developed. From the WaveShop website:
"WaveShop is a free, open-source audio editor for Windows XP/Vista/7. WaveShop is fast, lightweight, and bit-perfect, meaning samples aren't altered unless they need to be. Editing a portion of an audio file only affects that portion; the rest of the file is untouched. Blocks of audio can be cut and pasted without changing their contents at all. This is especially useful for patching a finished master without corrupting its dither. Waveshop's features include peak, RMS and spectral analysis, normalizing, fading, sample rate conversion, audio generation, and more, all with unlimited undo and comprehensive help."