I suppose this is two issues, really. I am trying to read from a FIFO,
specifically one written to by mpd, configured in mpd.conf with

audio_output {
  type "fifo"
  name "FIFO"
  path "/tmp/mpd.fifo"
  format "44100:16:2"
}

With mpd running I can see that the FIFO is there and I can read from it
with other tools/mpd clients.

(info "(elisp) Reading from Files") suggests I should be able to read
from a FIFO. From emacs -Q, insert the following into the scratch buffer:

(insert-file-contents "/tmp/mpd.fifo" nil 0 10 nil)

First issue: evaluating this yields

(file-error "not a regular file" "/tmp/mpd.fifo")

Second issue: changing the VISIT argument to t and evaluating:

(insert-file-contents "/tmp/mpd.fifo" t 0 10 nil)

breaks the display engine. "nil)" becomes invisible, the last "r" in the
scratch buffer message becomes fontified as a matching paren, and
hitting C-a at the end of the form takes me to the aforementioned "r".
I have attached an image.