There are different issues. You have the song copyrights, depending on whose repertoire they are singing. Even if there are songs that were Trad. in a strict sense, then the artists performing them are entitled to claim and be paid for their arrangement. If there are songs in copyright, then they would equally have to go through MCPS licensing for commercial use.   

Now though previously published recordings made after 1962 are in copyright for 70 years (and performers can re-claim the recordings if they are unused after 50) - unpublished recordings have a copyright duration of 50 years from their making "if a recording is not published or communicated to the public, the performers` rights last for 50 years from when the recording was made" Songs of course are themselves in copyright for 70 years after the death of the writer or arranger/s.