I can't find a video that accurately portrays its voice, so I'll just explain it.
Take the voice of a ~10 year old child, y'know, that time in your life when your voice is still high pitched but not quite at the point where it gets squeaky. Put, like, a metallic filter over it, right? And remove any trace of gender.
Oh, interesting thread. Since I doubt I'll find an example of a voice like this, Mailith just sounds as whatever voice she is trying to imitate but with an added accent. You can imagine this accent as difficulty to pronounce C, G, S and Z by filling the back of your mouth with water and attempting to speak in English. That's how a slime do.