If you hold of part 2 of Omer for mourning, thou mayest have music at the wedding; Notwithstanding, if your Sheva Berochos run into Iyyor, they are to be musicless, like someone marrying on say Tammuz 15th, whose Sheva Berochos after Tammuz 17th also have to be musicless.
As far as guests who observe 1st 33 days attending wedding of someone during last week of Nisson, at time of wedding thou art on turf of Baal[as] Simcho and hence follow his/her Minhog & are allowed to partake in music. That day still counts towards thy 33 days, as you couldn’t shave. But cannot listen to music in car on way to of from that wedding.
This being subservient to Minhog of Baal[as] Simcha only works for Availus of Omer or 3 weeks if invited to Yemenite wedding before Roch Chodesh Ov, but for mourner of a relative {i.e. Sheloshim & Yud Beis Chodesh}, this facet would not work.