I don't know about the beat of the music helping you open and keep the treasures...

Here's what I DO know.

1) They are one-time chests that disappear when you open one and its a trap.
2) There's up to 12 items...none of which are RLY great. There happen to be 2 Level 4 Key Spheres, 1 or 2 Level 3 Key Spheres, an armlet and bracer - one has fire, ice, lightningproofs on it (armlet) and the other has Stone, Beserk, and Poisonproof (or similar protective auto-abilities - bracer) There's also a Double AP weapon for Lu, and the coveted (but overall...not too useful) 99 Warp Spheres. The reason I say that none of this is all that great because you can easily customize all the weapons/armors, you can bribe for level 3 or 4 spheres, and the Warp Spheres just warp you to any location on the sphere grid (don't unlock anything.) O__O I suppose it might be kinda nice to have everyone know steal by warping them there...or any other great ability; but its not really necessary IMO. Having 2-3 characters that can cast a magic or support spell is generally enough anyway.
3) Its uber-rare: being able to open all of the chests without spawning a trap.
4) I'm never going to do it. XP