To one’s girlfriend, one might say:
(Kimi/Omae [wa],) Koe ga kawaii (ne).
Japanese かわいい ≠ (not equal) English/etc. kawaii
「カワイイ」を英語にする難しさ ハローキティはcuteそれともadorable? - ENGLISH JOURNAL
外国人が思う「Kawaii(カワイイ)」とは?Instagramから世界各国の「#Kawaii」を分析
But if you yourself are female, and because American girls are said to say often: "How lovely!" as a female/feminine language/expression, you could translate the sentence in question like:
(Anata,) Koe ga suteki (ne) or kawaii/ii(?) (wa ne).
Japanese kawaii (unlike English/etc. kawaii) can be said about anything feminine or child-like and adorable:
(Kimi/Omae/Anata no) So no fuku (= clothes), kawaii (wa) ne.
A Japanese comedian, the late Mr. Nikaku Shoofukutei, once said on a TV show, pointing to a female person among the audience:
"Kawaii neee! ... Fuku ga!" = "How pretty ... your clothes are!"
Japanese high-school girl students these days are said to even say about a middle-aged Japanese man: "Kawwwwaiiiii !," meaning he is lovable/honorable/respectable because of his diligence/hard-work etc..
Thus, nowadays, maybe:
kawaii = lovely or lovable.