Can I sacrifice an enchantment when it is going to be destroyed?
Yes you can sacrifice the Seal of Fire to do 2 points of damage. 

When the Lyrist activates its ability, that ability doesn't happen right away, it's put on the stack. While on the stack, you have the chance to respond, and so you activate Seal of Fire, doing 2 damage. 

Then Lyrist's ability tries to resolve, but since the enchantment isn't around anymore, it fizzles, but the costs stay paid (mana and sacrifice).