In the slow version of the voiceover, the sentence sounds very strange, like, "Dych achen" (is it just me or is the sound distorted?
It's because the Ivona (that's the company that made the Welsh text to speech) didn't record the phrase 'dych chi'. Instead they recorded the more standard written form 'Rydych chi'. The voice synthesiser then has to make the sound 'dych' out of two different ones in its library. This seems to work OK for normal speech but for the slow speech it pronounces the different sound files it's used separately.