A sound that's useless. Let's say, of no particular use. Nothing concrete. It has no function, no destiny. Humans produce them for no reason. Sound or music as a non applied art. As a comunicative "disfunction". It only appeals to the perception and this doesn't refer to anything else.
Since the beginnings of the 20th century the concept that the arts applied to objects of daily use should gather characteristics of utility remains. And that science of what's favorable is called design. There's good and bad design, only in relation to if it is, or if it isn't, to what it intends (or doesn't) intend to be. And that is a concept recently formulated, but that travels through the history of humanity, because nobody ever considered using a bow that shot poorly in front of a bison filled with potencial food for Winter. That paella and pistol contain their own processes and respond to a need. So do the tools to produce them. The setting of rules or general parameters, like ergonomy, correspond to the moment in which the possibility is given to mass-produce such solutions, after the massification and concentration of human necessities in cities and its necessity for  production. Those functional "arts" should vary and evolve without a doubt at the pace in which the necessities that make them exist vary or grow. We can talk about music. But if we talk about "music for", we find ourselves before an applied art. We have to discover those "musics for". To know the processes and needs that demand its existence. And let's not forget that design is the technology of doubt.
 

Aside from music understood as art applied to the arts, it is to communication, to consumption, to leisure or commercials, from the perspective of passive perception. Here below are some musical categories designed with functional objectives to which the reader can add all of which he or she wishes to discover.

Music to work harder or to wait more comfortably: the musical string in all its variants. Muzak.

Music to wait uncomfortably: contrary to the other, this music is conceived, for example, to demoralize the one who waits on the telephone and make him or her desist the call (loops of unbearable melodies on the phone during the wait).

Music to be bought. This is a very important category: music is composed to fill discs for those who have previously designed the ad campaign, the artist, his or her name and the cover, in that order.

Dance music: we know that it's entirely possible to dance without listening to music, but there exists music especially composed for dance. Perhaps, the musical "application" for other human activities or to make possible or alter physical, animical or mental states, may be something as old as music. Let's think that music alters and makes possible states without them having been conceived for itself, even more if it has been created with that objective. There's then a concept of music as substance and another as a motor, as a guide or as an assistent of body moviment (also of the mind).

Music for healing: music heals, but if one pratices, this is difficult only by listening to it (Active listening and participation are precise. The new age CDs  and others are a sham).

Music for slot machines, music for signposts, music for egg-laying hens, music to make plants grow, music to tame the wild animal, music to give the hour... (look for your own examples)
 

The production of functional sound for the arts (dance, theatre, cinema, video, installation, performance...), in comparison to the prior examples, searches the effect of syncronization  with the support that it accompanies. Sometimes, such syncronization is produced directlly in the machines, others are fulfilled manually, and in all cases, perform in the mind of the spectator.