Learn Turkish: Sen memnun musun?

In Turkish, personal endings like -sun (you) attach to the question particle rather than the word before it. So instead of saying memnunsun mu, you must put the personal ending at the very end, making it memnun musun.

Why is the question word musun here, but I've seen mısın in other questions?

This happens because of 4-way vowel harmony. The question particle looks at the last vowel of the previous word. Since the last vowel in memnun is an "u", the particle becomes mu, and the personal suffix for "you" also uses an "u", making the whole word -sun.

If -sun already means "you", do I need to include Sen at the beginning?

No, it is entirely optional. Because the ending -sun clearly tells us who the subject is, you can simply ask Memnun musun? Including Sen just adds a little extra emphasis, like asking "Are you pleased?"

I learned memnun oldum as "nice to meet you". Does this question relate to meeting someone?

Not exactly. The core meaning of the adjective memnun is "pleased", "glad", or "satisfied". While memnun oldum literally translates to "I became pleased" (and is used as a set phrase for greetings), Sen memnun musun? is just a general question asking if someone is happy or satisfied with a situation or thing.