Can you record any sound into the mic? or only from the software?

That other answer is incorrect, there is a built in mic and you can sample any audio you can output over a 3.5mm cable, though note the 35 puts all samples through the synth engine, the 33 records sounds as they are, so if you don’t want a change to the sample, the KO (33) is the one to get. 33 also has 4 voice polyphony, though the 35 still is cool, just better suited to syncing with others than standalone.

Why is this more expensive than buying directly from Teenage Engineering?

#1 - TE rarely has it in stock #2 - Not a lot of sellers want to sell at TE price because they lose money selling it on Amazon

Would this be suitable for a 9 year old kid?

Absolutely. It is very easy to understand and clear instructions. I have 2 kids around that age and they have no issues. Very little learning curve and plenty of YouTube videos on how to use it. The po-33 is more for recording sounds and making a beat where as the po-32 is synthesizing sounds and a little bit higher skill.

What is the function the “Lock” tab?

When you break off the Lock Tab, all patterns are frozen or "locked". To reverse this, you'll need solder the two pads by the lock tab together. You can learn more about the functions and features of PO-33 at [https://teenage.engineering/guides/po-33/en].