The artscroll talmud nedarim 51a brings shalmei nedarim, yaakov emden and einayim lamishpat who explain possible circumstances that could have permitted bar kappara to listen to rebbe’s wife sing at rebbe’s son’s wedding.
The relevant einayim lamishpat is here https://www.hebrewbooks.org/pdfpager.aspx?req=14885&st=&pgnum=101
The relevant shalmei nedarim is here https://www.hebrewbooks.org/pdfpager.aspx?req=37841&st=&pgnum=75
Can someone who reads hebrew tell me the reasons they bring for why he was allowed to listen to her sing? I don’t know where to find r’ emden’s comments but if you know could you tell me what reasons he brings as well