What is the sound of a baby's cry in Spanish (i.e., the Spanish equivalent for English's "Waah!" or "Wah!")?
According to this Fundéu list, the Spanish onomatopoeia for a crying baby is "bua":
- llanto de un bebé: ¡bua, bua!
The official dictionary (the DLE) does not include it as a word, though; but that's expected, as explained in the answer to this other question: Is there an official list of Spanish onomatopoeia?.
In Chile our onomatopoeia is ¡guaaa! This is because in the Mapuzungun language (the language of the Mapuche ethnic group), "baby" is called guagua, and later its use has been extended to all current Spanish speech, even in formal and cultured situations.
(I never thought that elsewhere the onomatopoeia was different, consequently I am very surprised to read here that the standard form is buaaa. In Chile, this form seems closer to the scare.)
What you are looking for is an "onomatopoeia".
You can find a little feedback here over the different onomatopoeias for the crying sound:
bua / buah / buahhh