Question: Why does Rav Shlomo Zalman Aurbach allow teenage girls to sing on buses, like on a class trip if they have a Jewish male bus driver? (Even if one or two girls are singing louder than the rest.)
A. The Mishna Berura (75:3:17) says one should be careful to not listen to a woman singing while reciting the Shema, even your wife, but her normal voice is fine. Other than while reciting the Shema a single girl’s voice is technically not considered forbidden, however now a days we treat her as a nidda who is forbidden just as a married woman or any other illicit relation, and even a non-Jewish woman, are all forbidden for men to listen to them singing. The concern is that it might lead to forbidden thought which possibly could lead to forbidden action.
Answer: While he is driving, he is too busy focusing on the road or distracted by his job so we aren’t concerned that he is really listening to the girls singing. (See Dirshu footnote 27.)