Who

1)Who is singing in that room? OKAY

2)Who are singing in that room? Not possible

In addition, who is a singular pronoun/adjective, so it takes a singular verb, but it agrees in number with its antecedent, so it expresses either a singular or plural meaning:

EX: Who found the dog? (Singular: What person? | Plural: What group of people?)
EX: He is the man who found the dog.
EX: They are the people who found the dog.

What would you ask the bellhop if you were disturbed by a chorus of drunken voices from the next suite? I would likely inquire, 'Who are singing in there?'

Who are your favorite authors?
Who are those bearded fellows in the daguerreotype?

Great addition, MM.

Though, given the linking structures provided, I dare say we've a strong case for 'exceptions'. But, then again, I may have missed your point.

Who are your favorite authors?
Who are those bearded fellows in the daguerreotype?

To me, "authors" and "fellows" function as the subject, irrespective of the question structure.

Mr M: What would you ask the bellhop if you were disturbed by a chorus of drunken voices from the next suite? I would likely inquire, 'Who are singing in there?'

JT: Yes good addition Mr M, but I find I have to agree with Casi. Though I can't rule out an with for every circumstance of English,

'Who are singing in there?' ,

sounds really strange to me.

Now, I'm in agreement with you about "Who are singing?" being awkward, but I believe "Who are going to come? supports MM's point quite nicely. That is, we should have read between the lines. Consider:

Pat: Everyone is coming to the party.
Max: Who (all) are coming? ~ Who are (all) coming?

If we assume "all" is implied, then "Who are" doesn't sound (as) awkward, it being

Now, I'm in agreement with you about "Who are singing?" being awkward, but I believe "Who are going to come? supports MM's point quite nicely. That is, we should have read between the lines. Consider:

Pat: Everyone is coming to the party.
Max: Who (all) are coming? ~ Who are (all) coming?

If we assume "all" is implied, then "Who are" doesn't sound (as) awkward, it bein

I too find 'who are singing?' (and 'who are going?') strange.

There doesn't seem to be a problem with the relative pronoun, which supports the point about known referents:

1. 'The people who were singing in the room...'

But even where we know at least 22 people are involved, 'who is' seems dominant:

2. 'I'm going to the football tonight.' 'Who's playing?'

I say, "But in first instance type questions, I still maintain [though I certainly can't state unequivocally that I'm right] that a "Who are ...? " sounds strange." ,

and in the next breath, Mr P <'who are those people?'> shows me I'm wrong. Back to the drawing board!

Could the difference be; the people are visible so in a sense it is not a first instance. Nope, that can't be

Thanks all for coming up with better support for me than I did.

I like the concept of 'in the absence of X, it defaults to Y'-- are there are other areas of the language where default is recognized?