I live in a city where there is only one orthodox synagogue. The Rabbi, who claims to be orthodox, recently conducted a pre Lag B’omer concert (two days before) whereby he sang songs (some chassidic) and accompanied himself on a guitar. I opposed the concert but, was overruled by the congregation. They said that the Rabbi was the final authortity. I discussed my opposition with the Rabbi and he said one instrument was allowed, but not more. I don’t want to put anyone put in unpleasant position. What should one do? It is mandatory according to Halacha to "Hocheach Tocheach". If I let it pass on the other hand, next year will come around?

The mourning period of the Omer is not as strict as the three weeks between 17th of Tamuz and 9th of Av; The Shulchan Aruch mentions the prohibition to shave, have haircuts and weddings. It is well known that all authorities forbid dancing, parties and songs though songs which are not of that nature, you can find many Halachic authorities whom will permit listening to, especially if they will bring Jews closer to there father in heaven. Your rabbi is the Halachic authority of his community, and I believe had reckoned his rulings taking into account all the different aspects of Halacha and of the community he is leading. If you think he is an incompetent rabbi (and this case you presented is not proof of that) you must discuss this matter with higher rabbinic personnel in your region who know this rabbi.