"Who is playing music next to the sea?"
Translation:Kik zenélnek a tenger mellett?
This sentence conjures up memories and images of the Beatles in their Octopus Garden!!
Questions words such as ki pretty much always need to be followed directly by the verb, in this case zenél.
You can say that, but it's rather uncommon.
Normally you say "Who is [verb]ing...?", regardless of the number of people you expect to do that. Note that this only counts for sentences in progressive tense. If you have a copula sentence with a plural object, you use "are": "Who are these people?"