Honestly, how his name is pronounced depends on how it is spelled in Hebrew. There are several different ways to say it depending on its context within the scriptures, which also influence the meaning behind the name (I recommend a short read called “Belial: A History” by Baal Kadmon for furthur information on the subject). Then we also have the Greek spelling of his name, which is Beliar (translating to “Baal of the Forest” or "Lord of the Forest, which leads me to the theory that the Greek Christian’s viewed him in the same light as Pan. Seeing as Belial was once viewed as being essentially the same as Satan this could explain where the goat horns and hooves imagery for demons possibly emerged). So, saying there is only one way to pronounce the name Belial is technically not correct and, frankly, I do not think he really cares how you pronounce it. His name is not who he really is, only what we use to identify him, much like how ours is used by others to identify us.
Baal Kadmon also has a majority of the same information in the book I mentioned on YouTube for anyone to learn from for free.