Because you are trying to find out about the subject of the verb you don’t need “does” or other auxiliary verbs For example: -Who saw Bob? -Martha saw him. But then: -Who did Bob see? -He saw Martha.

"Who plays the guitar ?" is the correct of your two. But if you add 'the' in the 2nd it would be correct as well. "Who does play the guitar ?". It's an informal version of "Who *can* play the guitar ?"

Because you’re trying to find out about the subject you don’t need auxiliary verbs such as does or did Look: -Who saw Bob? -Mary saw him. But then: -Who did Bob see? -He saw Mary In the second case you use “did” because you are asking about the “object” of the verb. PS: use “the” with musical instruments.

As Chris points out, it’s not that one is correct and one “sounds strange”. They are both correct and have slightly different meanings. “the” is optional in both questions. For example The guitarist is sick. Can the other music teachers fill in? None of them play (the) guitar. Ok, who does play (the) guitar? There has to be someone on staff who does.

I disagree with the other views. Depending on context, native speakers switch to the emphatic do. More information here: http://www.michellehenry.fr/emphasize.htm https://guidetogrammar.org/grammar/auxiliary.htm https://www.thoughtco.com/what-is-the-emphatic-do-1690590 and of course for a deeper explanation Practical English Usage 3rd ed by Swan, sections 158 and 184.