Can my opponent play a madness cost of 3 or below if "Brisela, Voice of Nightmares" is in play?
No, Brisela will not prevent your opponent from casting Volarden Pariah for its madness cost.
&quot;Mana cost&quot;, and by extension &quot;converted mana cost&quot; specifically refers to the cost printed at the top of the card. The relevant rules are here:


202.1. A card’s mana cost is indicated by mana symbols near the top of the card. (See rule 107.4.) [...]

202.3. The converted mana cost of an object is a number equal to the total amount of mana in its mana cost, regardless of color.



So, Volarden Pariah always has a mana cost of {3}{B}{B}, and a converted mana cost of 5.