The sound could potentially be 'detected' even if no one originally observed it.
For instance what if a sonic shockwave caused micro fissures in a rock which where then observed 1 million years later by a scientist. Even though nobody was there to observe the original event, there is still a record of the event - does this mean the event did not take place until it was observed? - for this to be the case it would require the detection of an event to affect things retroactively.
Alternatively you could say the rock itself is a type of detection device (given that it 'recorded' the event) in which case it could be said that any event/object that causes any type of disturbance to itself or its surroundings will ALWAYS be 'observed' via the effects it has upon its surroundings.
On another note, I once a Sci Fi story a very long time ago about a planet that tried to hide itself from observation in order to protect itself from being changed, does anybody know what it's called?)