Your friend asks whether you know a certain song. You don't recognize the name, so you want her to sing or hum a little bit of it.
How does it go?
Ask this when you want someone to sing the melody of a song that you can't remember.
You can also say that a song "goes like this":
A: How does it go?
B: It goes like this: La da da dee da da...
When you're asking about a song that you used to know, but don't know now, you can ask "How does it go again?"
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