Does Solemn Offering affect auras?
Yes. It can target any permanent with the Enchantment type, be it a plain old Enchantment, an Enchantment - Aura, a Legendary Enchantment Artifact, or an Enchantment Creature. Same goes for artifacts: it can destroy equipment, for example.

Aura is a subtype of enchantment, and something which says "target enchantment" doesn't care about subtypes, just that the permanent it targets has enchantment as a type. This is similar to the fact that something which says "destroy target creature" can target any creature, whether it's a Creature - Elf or a Creature - Ooze.