This is the 8885th most frequent Japanese word.
歌わ
"歌わ" means "sing" (negative or imperative stem form of the verb 歌う).
In this sentence, '歌わ' is part of '歌わない', indicating negation of the action 'to sing'.
彼女は美しい声で歌わないときでも魅力的です。
She is charming even when she doesn't sing with her beautiful voice.
Here, '歌わ' is used in '歌わないほうがいい', expressing a recommendation not to perform the act of singing.
もし雨が降るなら、外で歌わないほうがいいです。
If it rains, you'd better not sing outside.
In this usage, '歌わ' forms part of '歌わないわけがありません', indicating impossibility for the negated action of singing.