Silly question. Could you put one song in the middle of a play, and not have to call it a musical?

Yes.
A play can be whatever you want it to be!

For example:
John Osborne's "The Entertainer" about a seedy musical hall performer is full of songs (and dances) but no one ever calls it a musical.

Personally, I absolutely and firmly believe that a play is whatever Sonia Friedman wants it to be!!!!!!!!

quote:

A play can be whatever you you want it to be

...subject of course to the infallible, unerring and unquestionable judgement of Sonia Friedman.

I wish this to be clearly understood.
Thank you

Too late Lewes!

Another One Bites The Dust ...

Cheers. I agree!
Also, music would be so important imho but maybe its a cheats way of shoe-horning some emotion into the piece ? Incidental music wouldn't work for a play, but things like 'Buffy' (sorry) where you have wonderful ballads strung into the middle of the story quite naturally, which lead you into the next scene - could work I guess.

Yes, right.
Taking up Adman's point;
none of us know(and of course C4 hasn't seen fit to enlighen us)
exactly what it is TPTT are looking for in a play;
Trad (West End); Something new; or what?