Use the word baa to describe the sound a sheep makes. A lamb might baa for its mother if it finds itself alone.
Every language has words that mimic the cries and noises animals make, and in English sheep and goats baa. In Dutch, sheep say bè bè, and in Japanese they say meh meh. The imitative word baa, first recorded in the late 1500s, can be either a noun or a verb, depending how you use it. Baa makes a famous appearance in the well-known children's song "Baa baa Black Sheep."