There’s nothing wrong with innovation, so long as it’s in the spirit of the occasion. Musical instruments, dancing, whatever, just because it’s new doesn’t make it wrong. The question is only what kind of music, and I do not accept your assertion that the music is “frolicking”, “disco-like”, “wild”, etc. If there is indeed a shul where the atmosphere at selichos — with or without instruments — is such that you could mistake it for a wedding, let alone a nightclub, then I condemn that particular instance. That would not justify extending the condemnation to all musical selichos, the vast majority of which, I’m sure, do maintain a slichos-dige atmosphere during selichos. Before and after is a different story.
But those who are upset with the whole idea of simcha at such a time have forgotten וגילו ברעדה. There must certainly be רעדה, and without it it’s not a slichos, but one must not forget the גילו either.