But, how did we solve it? To see if it made a sound then we would have to be there to hear it. Which would mean that someone was around which caused the sound.
Sound is really just vibrations made into electrical signals which our brains can make sense of. Since these vibrations occur wether or not human ears are present to convert it into recognisable 'noise' doesn't affect wether those vibrations occur.
Does gravity exist if there is nothing around to feel the effects?