。
She sits as if entranced by the music.
Literal
She [topic-は] music [by-に] was-enchanted [as if-かのように] is-sitting.
魅せられる is the passive of 魅せる ('to charm, to enchant') — and a fine example of how Japanese uses passive constructions for emotional reception, where English would typically use an active 'is enchanted by.' The に marks the agent doing the enchanting, here the music itself. Wrap it inside ~かのように and the perception layer ('she looks as though...') is added: not literally enchanted, just sitting in a way that suggests it.