the bleating of sheep
咩 represents the bleating sound of a sheep, essentially an onomatopoeic character for the vocalization of sheep.
咩 combines 口 (mouth) with 羊 (sheep) to express the bleating sound made by a sheep. The exact historical development is uncertain, but the semantic composition is straightforward.
A mouth 口 next to a sheep 羊: picture a sheep opening its mouth and letting out a bleat.
For ビ, imagine a sheep bleating 'beeee' like the English word 'bee': the 'bee' sound cues ビ while the sheep bleats.
—