I want to post this here because I couldn't find this solution anywhere on the forum or online. This worked for me and I hope I can save someone some time.

I had created a mod and accidentally overwrote Vorstag's dialogue a little bit. I was really worried because this was affecting the vanilla game even with a load from before the mod.

The answer is to go into Steam>Steam Apps>Common>Skyrim>Data>Sound>Voice>Skyrim.esm. You will see only a few folders, namely the ones of vanilla voicetypes you have affected (in my case "MaleEvenTonedAccented"). Just delete those folders and it will restore the vanilla dialogue.

As far as I know, only audio files that you create will go into the Sound>Voice folder, and will not harm the game if deleted. Unless you have extracted the voice files from the bsa file and deleted the bsa file.