、。
Shh, you're going to be heard, I said.
Literal
Shh, be-heard-[completion-ちゃう] [insistence-ってば].
しーっ is the onomatopoeic 'shh' with a drawn-out vowel and a final glottal-stop ッ for emphasis. 聞こえちゃう is the casual contraction of 聞こえてしまう 'will end up being heard (regrettably).' ってば is a sentence-final particle of impatient insistence — 'I told you already / come on' — often used when the speaker is frustrated that their previous request or statement wasn't heeded.