Which is why you really want to listen to someone saying the name, rather than trying to figure it out from a pronunciation guide. I don't know about you, but I can't imagine what "L" + "th" + "hiss" should really sound like.
My next query was intended to look for the category of names (rather than just the single name Llewellyn) and also look for pages that would have recordings on them.
What's the most common way of describing pages with sound bites? I'm willing to bet they ALL say something like "hear these names spoken aloud" (or equivalent language). So I chose to use the term 'hear' and the category 'Welsh names' as in this query:
In yesterday's comments, Hans points to the http://www.pronouncenames.com/pronounce/llewellyn page, which has a nice "standard English" vs. "Welsh" pronunciation side-by-side. As Hans' solution points out, there are multiple ways to find a good answer to the original question.