Question: We attended a Bar Mitzva where they played music even though it was during the omer. Was this okay?
Answer: The poskim (Aruch Hashulchan OC 493:2; Igros Moshe YD 2:137) write that one mustn’t listen to music during the omer. R’ Yitzchak Yaakov Weiss (Minchas Yitzchak 1:111) writes that this minhag is so important that one wouldn’t even be able to play music at a seudas mitzva.
R’ Ovadia Yosef (Yechave Daas 1:45; 6:34), however, writes that one may play music at a seudas mitzva during the omer, be it a bris seuda, bar mitzva or siyum, etc. R’ Ben Zion Abba Shaul (Ohr Letzion 3:17:1) also allows playing music at such occasions, writing that the simcha of the mitzva overrides the minhag not to play music.
R’ Moshe Feinstein (Igros Moshe OC 2:95) allows one to play music at a sheva berachos for one who got married on lag b’omer. Elsewhere, (ibid. EH 1:98) he writes that while one can make a party for a chassan and kalla who had returned to town after their sheva berachos, one couldn’t play music then.
R’ Moshe Sternbuch (Teshuvos Vehanhagos 4:128) writes that an ashkenazi may attend and participate in a sefardi chasuna during the ‘three weeks’ even though they wouldn’t make one then themselves. Thus, it would seem that an ashkenazi may attend a sefardi simcha in which music is played.
In conclusion, there is a machlokes as to which occasions one would be allowed to play music at. One could attended a simcha where music was being played, even if it was their minhag to rerfrain from playing at such events.