Symbolism, Imagery, Allegory
It doesn't figure all that prominently into the story, but the player piano in the corner of the Silenus Restaurant adds another note of randomness and absurdity (even comedy) to Conrad's dark universe.
The thing starts playing random songs at random times, always when the Professor and Ossipon are having their conversations and entering or leaving. As the narrator describes in Chapter Four:
The lonely piano, without as much as a music stool to help it, struck a few chords courageously, and beginning a selection of national airs, played him out at last to the tune of "The Blue Bells of Scotland." (4.125)
The instrument provides almost a comedic backdrop to the supposed epic-ness of what the two anarchists are discussing. The narrator takes great pains to imply that there is no rhyme or reason to when the piano, which "cease[s], as abruptly as it start[s]" playing (4.3).