system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: What's happening when I "sing" in my head?

I find that I can play songs in my head, and that it feels like I can hear music (or even my voice singing along) even though I know there's not really sound.
gpt
Ok like your 5.....

Your brain is made up of lots of different parts, one that sees things, one that listens to things, one that feels things and all sorts of other things.

All these parts have memory, when you hear something its remembered in the same part, and when you remember something it plays with the same bits that heard it in the first place. Since the process of remembering and hearing use the same parts of the brain. It feels the same to you.

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ELI25

Memory is in the synapses (the strength of the connection between neurons). When one neuron fires it may or may-not cause another to fire (depending on the strength of your memory). The synapses that hold memory are the same synapses that were used in the first place to hear the music. If your memory of a song is good enough then something could trigger a small pattern of neurons which could propagate across the synapses with strong memory causing a sensation of hearing music.

The reason this dosent sound exactly like real music (you can tell the difference between actual sounds and the stuff in your head) is because many more neurons will be involved in the process of listening to sounds than the remembering of sounds.