While in principle anything that can produce sound could be considered a musical sound device, a musical instrument stands out by the ability to reduce a complex physical system of sound generation to obey the temperaments of a musician. Much like a piano, an organ, a guitar or a synthesizer the instrument rewrites the underlying physical system to an intuitive language. This language that is set by the instrument creates the unique emotion by which it is distinguishable.
Another fact that distinguishes a “great” instrument is probably best stated by the Bushnell's Law: “all the best games are easy to learn and difficult to master”.
Taking these two facts into account while conceiving what could be a musical instrument, it is possible to hypothesize how the instrument and its language should be learned, played and mastered.
One can imagine that for a person holding the instrument for the first time, the basic level of the instrument’s language should allow to easily find the notes on the schema of the instrument. Much like how the notes are laid on the piano keys or different frets and grips on a guitar there is basically not a lot more that is needed to learn or even maybe master the instrument. For the most part of this musical journey a minimal amount of physics is even only implicitly needed.
Yet on the other end of this musical journey lies a very complex physical system that produces these tones and frequencies. Concepts of acoustic systems of any instrument such as the proportion of length and notes, frequency to a resonant body, all the way to the dynamics of exciting a string to a note, only become evident with progression of musical talent.
Therefore what makes a musical instrument a unique object is the transcendental relationship between the complexities of physics and simple intuition of the senses.
Much as the notion of realizing lengths and proportions of strings by listening for their frequencies, through language of tones to lengths the well-tempered chaos sequencer sets to illustrate notions of chaos by the virtue of the underlying emerging structures through a high number of repetitions and an intuitive visual aid.