I looked Greater Rage up on Wiki and interesting enough Wiki does not indicate that Greater Rage gives the sonic bonus at all; must be an addition made specifically for Layo.
I know that the magical damage given by the clerical spell Battletide
is not affected by damage resistance or immunities (ie, magic resist 5/-). That's because its considered a "damage increase affect". Maybe this sonic damage is similar to that.
EDIT: Ah, just read your other post where you already noted that Battletide magical damage is not affected by damage resistance, so maybe you already knew that.