The issur is on live instrumental music, is it not? Acapella singing is not live music, no matter how much the voices sound like instruments. So the question then becomes, what is the intent of the issur? If it is to avoid the simcha of hearing beautiful sounds that lift the spirit, then it would make sense that even acapella is assur at those times. But if the issur is strictly on the musical instruments, because instruments bring about a certain level of enjoyment that even voices cannot, so it would make sense that the issur is only on hearing live musical instruments, but not on the acapella singing.
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