There is a tradition of 'silent' pieces of music, of which the most famous is John Cage's 4' 33'. However, that was not the earliest example: the first was probably the humorously titled 'Funeral March for the Obsequies of a Large Deaf Man' composed in 1884 by Alphonse Allais. In recent years, new technologies have enabled the creation of all sorts of music that is not exactly silent but extremely quiet, including sounds that sit outside the range of human hearing. It may seem strange, but these sounds are felt by the listener, even if they cannot be obviously heard: their absence is as strong as their presence.
Make a piece that consists only of human breathing. The level should never rise above 10 dBPSL. Try to work towards as much silence as possible, without losing atmosphere. The aim is keep the listener's attention with the least possible information. At what point does the listener lose contact with organised sound and connect with environmental sound? Can this be controlled?
The use of human breathing will keep this piece within the (just) audible range. The volume levels during playback need to be carefully controlled.