！
She really does sing with a beautiful voice, doesn't she!
Literal
She [topic-は] truly beautiful voice [with-で] sings [right?-ね]!
ほんとに is the casual contraction of 本当に ('truly, really'); the colloquial dropping of う makes it feel spoken and warm. The particle で here marks the means or instrument — 'with [a beautiful voice]' — one of で's many roles. Sentence-final ね invites the listener to share or confirm the observation — not a true question, more 'don't you agree?' The exclamation mark and admiring tone push this clearly into spoken register.