My advice is don't use Holocron Toolset, it's not advisable to use it for publically released mods, it adds and alters entries that aren't necessary in many different files when editing them, it's handy for the 3D view, that's about it in my opinion, it's still in Beta effectively.
You probably just need to use the other entry, VO_ResRef I believe is the .lip file and Sound is the actual sound file, there is a "Sound Exists" checkbox, but I checked on a file I am using, which doesn't have that ticked and the voice still plays.
If as you described other files are working in the entry you are trying to use for your files, then it's the files.
You are using the "encode_vo.bat"? or are you entering the command line arguments for SithCodec?
Perhaps you could try them unencoded?