This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
The exact historical development of 聠 is uncertain. It combines 耳 (ear) with 并, but the original semantic and phonetic roles are unclear.
With 耳 (ear) on the left and 并 (a shape suggesting joining or combining) on the right, imagine ears joined together, perhaps listening intently or closing off sound. Without a clear meaning, this visual image is only a mnemonic hook.
For ヘイ, picture a pair of ears (耳) hearing a loud 'hey!' sound: hey -> ヘイ, and the ears are joined together listening.