(For anyone not aware, this is related to my guide Making a Unique-Voiced Follower in Skyrim.)

Given that you’ve checked your work six times (!) against the guide I’m pretty sure you’re not making some simple mistake.

I imagine you’re using WAV files (as the guide instructs)? I have heard that the Creation Kit and/or Skyrim can have problems with WAV files. Specifically, from this page, I have read:

The game engine occasionally fails to playback audio files saved in the WAV/XMW format for reasons yet to be deciphered. To workaround this bug, FUZE equivalents of the voice assets need to be created. This can be done by converting the source WAV files (when applicable) to the XWM format, and subsequently to FUZ by pairing them with their LIP files.

I talk about FUZ files in part 10 of the guide, under the section “Other Archiving Information”.

Hopefully this information will give you something to go on.

Good luck!