chirping, birds singing together
嚶 centers on the sound of birds vocalizing together: chirping, singing, or calling in a group.
嚶 combines 口 (mouth) with 嬰, which likely contributes the sound and may relate to encircling or binding, suggesting birds' calls intertwining. The exact historical development is uncertain.
The left side 口 is a mouth, and the right side 嬰 looks like a complex nest with a woman and shells. Imagine many bird mouths around a nest, all chirping together.
For オウ, picture a flock of birds singing 'oh!' in unison: 'oh' -> オウ, as they chirp together.