I think a capella is music, especially from back in the day (afterall, we do have Beatle fans here, so…)
A singers vocal cords are trained with little difference than a guitarist or violinist is trained to created a musical sound.
Also, singing to a singer is no less enjoyable than it is for a musician to play an instrument.
I am guessing a capella is ok during sfira because you are not making an external, independent object create music.
I dont listen to it during sfira on Kol Brama for the same reason as soliek
Good question to think about, though.