Animusic is a series of computer generated animation of imagined mechanical devices that plays instrumental music.
This is one of their best:
This is fantastic. Such a machine is certainly possible, but not as depicted exactly by the video. For example, when balls hit the strings or drum, even when precisely controlled, their ending path is not predictable because the hit target is still moving, such as cymbals. So, the balls will not all neatly fall into their catch hole.
This can be mended by having the balls simply fall on the floor. Stray balls colliding with instruments can be avoided by considering all possible bounce paths. Also, the instrument itself, such as cymbals, can be designed so that it doesn't move in a uncontrolled way. With the floor approach, the floor needs to be made soudless. One simple way is to make it a “bottomless well”, for example, suppose this chamber is hosted on the floorless 10th floor. I'm sure there are other methods, such as watered floor, or a slanted trampoline-like catch that bounces the balls silently to the side…
Also, if we were to design such a mechanical device to play midi music, and suppose the goal does not need to be visually pleasing, then such mechanical device'd be much simpler. Player piano, Music box, are examples.
Also, note that some of the devices depicted is not flexible enough to play any other composition. For example, in the beginning of the video, a ball hits a string, then it bounces off onto another string, then bounces off onto a drum. This means, when a ball fires, it necessarily makes 3 notes in sequence. Most music isn't sequence of 3 notes. Of course, this virtual device is designed this way to make it visually and mechanically stunning. This aspect is actually the main attraction of Animusic.
So, in designing such a virtual music device, one could make it as logical as possible, such as the mechanics of a piano, where strings are simply laid out by length, flat on a plane, with arrays of hammers and dampers for each string, ready to spring into action when receiving a signal. Or, the strings can be laid out in a very creative way, without consideration of practicalities, but still mechanically feasible. For example, instead of hammers hitting the string, it is plausible to have the string move to hit the hammer. Even though, this would create a lot complexities in the device if we were to actually make it, but the point of Animusic is to make visually and mechanically stunning instruments.
Also, instead of one set of strings laid out flat and let the melody determine which string gets hit, we can instead consider a given piece of melody as fixed, and have the devices change to fit the music. A simple example is to have just one single hammer, and let the strings move to it's target position. Also, the strings need not be layout flat on a plane, but it can be laid out in fancy ways, such as circular.
Or, a piano-like instrument be made with one single hammer and one single string. The string's length is variable and mechanically controlled, so it can actually very length for different pitch within a fraction of a second. (this is certainly possible with today's tech) Now, because a piano's pitch is not simply length of string, and for different sections you actually need different string materials, so, instead one single set of a hammer with a string, we could have multiple of them, each focus on a different pitch range. Multiple set also allow harmonics.
Similarly, this applies to other instruments, such as drums, strings, wind instruments. There are a lot potential for creativity.
In the Pipe Dream video, there is one prominent example of flowing instrument over fixed music idea. Near the middle of the video, various sized marimba bars moves like a train over a rail, to be hit by a regular sequence of balls dropped into one spot. (as opposed to, fixed marimba bars in a plane)
There are some principles though. For example, the devices should remain mechanical and plausible. That's the charm of it.