If you're not the least bit interested in listening to their music -- just so you can keep up with them -- you're going to have to give up the insecurity based on not feeling like you fit in during the times they're cracking up over lyrics, and belting out the words. Accept it, and maybe you can introduce them to the music you like. Friendships are a balancing act -- like most relationships. If they tilt too far to the negative, it's time to re-evaluate whether it's worth the effort. But you seem to truly like this great group of new friends, and they seem to be supplying you with other friendly input to make it worth hanging out with them. Compatibility is what makes friendships work for the long haul.