Use MKextTool (download here in forum downloads or directly @apple) and unpack the extensions.mkext container. Remove or replace the kext, use MKextTool to create new Extensions.mkext container.
You actually don't need .mkext. You can keep your kexts in your /Extra/Extensions sub-folder. Works fine. But you shouldn't mix both. Solution which can cause issues.
AFAIK, you need some kind of xxxHDA.kext but it depends on your system which one fits.