I the world we live in, most children’s toys are battery operated and loud. Some, like the music set, are just loud. For the battery operated ones, this is a good trick that is easy.
Inside the toy, there is a speaker that delivers the noise. I don’t know why they come factory set at a deafening level, but they do. One solution is to use old batteries. That lowers the volume, but it also slows the music down so it sounds even worse.
A better trick is to put a piece of clear tape over the speaker. That is it. The same tape you use to wrap the noisy Christmas presents will tone them down a notch or two.