What is the ruling on someone who entered a gold shop and heard the Qur’an being recited in a very faint voice, then began speaking with the seller in a voice louder than the sound of the recitation? Is he considered a participant in the wrongdoing?

Playing the Quran without listening to it is not permissible. It is considered makruh (definition) (disliked) by the Hanbalis, and Ibn Aqil views it as haram (definition) (forbidden) if it implies disrespect. You incur no sin for disapproving of the matter in your heart and intending to offer advice. However, you should have gently advised the man that the etiquette of the Quran necessitates listening to it, and that such an action suggests turning away from it.