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It depends on the toy and my child. I rotate my son's toys regularly anyway (he's 17 months old), so if he only played with it intermittantly, I'd probably put in hibernation on a fairly regular basis. If it's something he adores, and he plays with every day, and the annoying music soothes him - I'd suck it up. Everything's phases for kids anyway, so eventually his attachment to the toy would pass. Hopefully sooner rather than later!
For my nephews we cover the speaker with a couple of pieces of tape... it significantly reduces the level of noise from the toy so it is not nearly as annoying and it does not bother my dad's migraines as much
i would just turn the volume down on the noisy toys. otherwise pull the battery out, but you said you were unable to recover the battery. the first poster had the right idea by rotating the toys.
seems leapster toys are obnoxious about the loud 2 volume settings, but the low setting is loud enough thankyouverymuch. so i super-glued a chunk of a plastic stir-stick into the spot between low/hi volume. next day my twins could only off/on low. no more horrendously loud toys in my house....i feel your pain:)