My son has a few toys that are so loud and piercing that they hurt my ears (emergency trucks with sirens, an aeroplane flies around in a circle on an articulated arm, and a toy musical piano). My ears have had almost forty years of abuse, and I find them unbearable; I hate to think of the potential for damage to my son's ears. I don't understand how toys can be made and sold with such deafening noise levels.
I know enough about electricalness to know that I could just open up the toy and snip the wire to the speaker. But what if I don't want to mute it? I have considered covering external speaker grills with tape, but my son would probably just remove the tape. Is there something I can I do – discretely, from inside the toy – to an electronic speaker to quieten it without removing the sound entirely?
If you have a solding iron and know something about electronics, you can put a resistor in series with the speaker, reducing the output power (part of the power that would be converted in sound will be converted in heat by the resistor)
It there's room inside the toy, you can try to reverse the speaker, so that it's pointing inside the toy and not outside
(Philip answer) You can put noise absorbent materials in front of the speaker: cotton, foam, tape...
(Dotacohen answer) You can put some material to compress the speaker, so that it won't move freely
You can damage the speaker by making some holes / cuts on it, but it won't reduce the noise that much as it will distort the sound
You can damage the speaker by droping some glue over it. This will make the diaphragm heavier and will produce less sound.