I played the sound several times and kept hearing “ĉiron” not “ĉielon”. I guessed that it wasn’t right - unless it had been in a science fiction story or something similar (ĉiro = tendril) - but just couldn’t work out what it could be.
I like it that the speech (or my hearing or the sound reproduction on my ipad) is not always perfect. It gives me the chance to try and think in Esperanto to try to work out the meaning. This time I failed but I am improving. Some things that I couldn’t hear properly in the course last year are now obvious.