Lute, Guitar and Tambourine are the ones people actually use.
Rattle actually has some limited utility, but the situations where it might come in handy are pretty few. The last time I saw someone use it was in our first couple of runs in the Zombie room in GCH (the ones with the box-stairs, torches to be lit and the mass of zombies that have to be kited). Our tank was learning the kite, and he was taking a pretty heavy beating, and having rattle played constantly helped a lot. Since everyone and their cousin has a few rattles sitting around thanks to the random music boxes, it was easy enough to juggle cd's.
Saxophone is only useful for playing mariachi band when you're all bored cause the tank dc'd again and you have to wait ten minutes for him to restart. It debuffs the mobs, but I have never seen anyone actually use it because the range is too small- by the time they get the debuff, you've already pulled. Sounds pretty good in the mariachi band though.
Casters should go ahead and level Tailoring and Carpentry to 33 for tambo and guitar. Melee should go ahead and get Alchemy and Tailoring to 33 for lute and tambo. Everyone uses those constantly.
Like LadyMacV said, in a 6 man, it's usually doable to just group up off the sides in the boss room so each group of casters and melee can get the proper buff. In some cases instead of doing that you can make the party a raid and split into two parties inside the raid, as long as you have someone with quick fingers to move them all back for group heals as soon as music has been played. That's when druids come in super-handy, as they can heal two separate parties as efficiently as one. But we've done this with a priest before. It just depends on the layout of the boss room. Some boss rooms have a no-aggro zone along the sides that allows easy party splitting, and some don't.
Music buffs have a pretty short range (100 iirc), and they will only effect people in a party inside a raid, so either you scoot out of 100 range of each other or you literally split the party up into separate parties in the raid and stand wherever you want.
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