Just to settle the matter of what language is where:
PHB, page 306 (Glossary) "...Any
extra damage dice...are not rolled multiple times..."
PHB, page 114 (Weapon Qualities) "...
Extra damage over and above a weapon's normal damage, such as that caused by a sneak attack or the special ability of a flaming sword is not multiplied when you score a critical hit..."
PHB page 134 (Damage) "......Exception:
Extra damage over and above a weapon's normal damage, such as that caused by a sneak attack or the special ability of a flaming sword is not multiplied when you score a critical hit."
PHB page 140 (Actions in Combat) "...
Extra damage over and above a weapon's normal damage, such as that caused by a sneak attack or the special ability of a flaming sword is not multiplied when you score a critical hit..."
DMG page 26 (Behind the Curtain) "The reason that critical hits
multiply all damage rather than just the die roll...
Multiplying all damage, the roll and the bonuses, makes..."
DMG page 221 (Weapons) - "Some magic weapons deal additional dice of damage. Unlike other modifiers to damage,
additional dice of damage are not multiplied when the attacker scores a critical hit."
SRD (Hypertext SRD) "...Exception:
Extra damage dice over and above a weapon’s normal damage are never multiplied. "
SRD (Official, COMBAT file) "...Exception:
Extra damage dice over and above a weapon’s normal damage are never multiplied."
Draw your own conclusions.
