Entrance to the chamber: an iron door. If the characters spend time examining it, they will note that it becomes very hot to the touch, then 30 seconds later very cold to the touch, then back to hot 30 seconds later, etc. Listening at the door will also reveal a repeating pattern of five low-frequency musical tones.
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Near the entrance on each side is a statue: one statue of a red dragon, and one of a white dragon, both facing the door. They seem to leer at the delvers as they enter the room, grinning. Once every minute, the red dragon statue spews a blistering cone of fire, and the frost dragon blows a biting cone of frost.
Beyond the statues, where the room narrows, a stone slab 4' tall blocks the way. The slab is hanging from the ceiling, supported by five thick steel cables of varying thickness, starting with the thickest on the left and the thinnest on the right (similar to what one would find under the hood of a piano or on a guitar). Higher up, out of reach, mechanical hammers strike the steel cables to create the pattern of musical tones.
If the characters look closely, they'll see that the stone slab is already a few inches off the floor, but this doesn't leave enough space for them to crawl through. Also, the cables are thick enough and grouped together closely enough that characters cannot squeeze between them to climb over the slab.
A stand with the next magical instrument they must collect is clearly visible on the far side.
Solution: Careful examination of the statues will reveal that the heads are not fixed into place; they can be swiveled around and aimed. Aiming the red dragon head at the cables will cause them to expand, lowering both the pitch of the musical tones and the slab of stone, not helping the characters at all. Aiming the white dragon's head at the cables, however, will cause the cables to contract, thereby shortening their length, causing the musical tones to rise in pitch, and raising the slab of stone to a height sufficient to allow the characters to crawl under it.