Does minor illusion require prior knowledge of the sound/image in order to create it (similar to Wild Shape requirements?) Or is it more like polymorph that doesn't have any restriction and I could emulate the sound of a specific person or the image of something I've never seen?
Any Object/Sound You Can Imagine
If the illusion created by Minor Illusion required the caster to have seen the object or heard the sound they are creating before, the spell would say so (as it does for the description of Minor Conjuration on p. 116, bold added)
MINOR CONJURATION...its form must be that of a nonmagical object that you have seen.
Minor Illusion's text contains no such restriction.
Although anything you imagine is some combination of, or change to, things you have perceived (since with no frame of reference we couldn't imagine it to begin with), you need not have experienced the precise thing you create (ex: you could create an image of a 4' tall statue of a manticore, even if you'd never seen a statue of a manticore at all, as long as you know what a manticore looks like).
Yes, sort of. The illusion would necessarily be of what the caster imagines the image or sound to be, and he may or may not get it right. I would probably run that as the caster having to make a Deception roll to make a convincing illusion of something he has never seen or heard, and perhaps with a higher DC to meet if the target(s) have seen or heard the real thing.
So, as an example, suppose a spellcaster decides to make a sound like a dragon's roar, to scare a bunch of goblinoids, but he hasn't heard that before. He kind of knows what roaring sounds like, though, and he figures the goblinoids probably haven't heard a real one, either, so he gives it a shot. I'd have him roll a Deception skill roll, with the DC being the Passive Perception of the goblinoids in question. If he happened to be a Wizard that specialized in the school of Illusion, I would probably give him his proficiency bonus on that roll, if he didn't already have proficiency in Deception.
So, to fool the goblins, he needs a 9 total; to fool the hobgoblins requires a 10; to fool the bugbears is also a 10, except the bugbear chief is an 11. But say the hobgoblin warlord actually heard a dragon roar once -- I'd add 5 to the DC for him, so the spellcaster would need a 15 to fool the warlord. So, if he has a +4 in Deception, and rolled a natural 6, that's a total of 10, and they would all hear a great and frightening roar, except the bugbear chief and the hobgoblin warlord, who would squint funny and say to themselves "that doesn't sound right", and start yelling at all the cannon fodder that they should charge anyway.
If you want to be more detailed and "realistic", and less predictable, you might have them all roll their Perception instead of using the Passive to see if they get fooled.
So what happens then? It's only a cantrip, and only a noise, so it shouldn't turn the whole battle, and shouldn't be as powerful as, say, getting surprise on the whole group, or casting an actual Fear spell, so I might rule that anyone who got fooled takes a -5 on his initiative for one round after the roar. Just makes them hesitate for a couple of seconds, and maybe that helps the PCs in the fight a little bit, which is about right for the effectiveness of a cantrip.