I did a bit of a disservice by posting only electronic pieces. Avant-garde does NOT have to be electronic. There are a huge number of avant-garde compositions for conventional instruments and orchestras. Often conventional and non-conventional are mixed in orchestras.

Varese composed a piece for orchestra that utilized a rope being pulled through a hole in a fiberglass tub. George Antheil's "Ballet Mecanique" from 1915 featured door buzzers and airplane propellers among the orchestra. In fact, I believe that Antheil vehemently insisted his composition was NOT avant-garde. Charles Ives wrote avant-gard pieces for orchestras as did Harry Partch who also wrote simply for voices and who created his own instruments so he could compose in microtonal scales.

Below is an avent-garde piece written by Varese for the flute. He wrote it for a friend to commemorate his new platinum flute. Varese entitled it Density 21.5 which is the density of platinum. Not infrequently, you'll hear the more adventurous souls tackle it at flute recitals.

Density 21.5