1 chirp, tweet, cheep, squeak, twitter, pipe, chirrup:
The birds peeped excitedly when they saw the cat.
2 chirp, tweet, cheep, squeak, twitter, pipe, chirrup, chirr or chirre or churr:
The night was filled with the peep of the frogs at the pond.
3 sound, complaint, outcry, protest, protestation, grumble, murmur:
The students didn't let out a peep when ordered to remain after class.