Hey Koji :) As Périne said, "jaser" is the sound procuded buy birds. But, in slang, it also means "to talk shit" kind of, like, it means to talk too much, to talk nonsense. Can also mean to speak while being too loud. All these kind of negative words. What was the context ? Can you share the sentence in which you saw it ? Hope that helps ;)

Hi Koji! In French the verb "jaser" refers (or more exactly referred because this word is now a bit obsolete) to the different sounds made by the birds (squawking, whistling...). So in this sentence we can imagine that the mandolin makes more or less the same sounds than birds. I hope this helps :)