Rashi: When they heard the music, which was a sign to bow, all the nations fell and bowed.

Malbim: It did not say Sha'asa (like the previous verse). Also, Sumponeyah was omitted from the instruments. I say that the officers were Sheluchim of the people to bow, but also the people in their cities bowed to show consent. It was the same day, but they could not coordinate to be exactly the same time. They played music in the cities, and then they bowed. However, Sumponeyah is an expensive, esteemed Kli. It was only in the king's palace; they brought it to the valley of Dura, but it was not in the cities. Perhaps the people bowed on their own to show consent to the king, or he commanded them. The verses did not explicitly say so, but it is included in the command to the officers.