Specifically I’m wondering about electronics that play back short noises or songs, like baby toys. Why would the sounds slow down or get distorted when the batteries get low?
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People have talked about the frequency issues, but there is another, inherently analog reason for distortion.
The sound is produced by sending an analog signal to a speaker. No matter how digital everything is, the last stage is an analog amplifier that generates a powerful enough signal to move the speaker cone.
To keep it simple, lets imagine you are playing a single pure tone. That tone will require a sine-wave signal sent to the speaker. A tiny, low-voltage sine-wave will be sent to the amplifier, which makes a bigger, higher voltage one. The amplifier’s output shape will track the input shape, ideally perfectly. However, if in order to generate a loud enough sound, the output voltage has to go higher than the supplied battery voltage, it won’t. The top of the sine wave will be cut off, because the amplifier cannot generate an output higher than its supply voltage. A cut-off sine wave is not a pure tone, it is a very buzzy, distorted tone.
The same principle applies to more complex sounds. Insufficient voltage to the final amplifier will generate distorted sounds.
High-end gear won’t do this, as the battery voltage is regulated and until the battery is too dead to do much of anything, the amplifier will get the right voltage. For something like a baby toy, the manufacturer will not invest in regulating the output voltage of the battery, and you will get distortion as the battery voltage falls.
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