Does your cocktail hour take place in three different rooms? Is your reception hall massive? Are you having a ceremony onsite too?
Let’s say all the above was true. Have you asked your DJ if they have enough speakers? If they don’t, he may decide to leave the bar area without music, or only cover half of the reception room. Or after the ceremony, you might see your DJ moving speakers from the ceremony room over to the reception area; it’s never good to mess around with the audio right before introductions are about to happen (too many things can go wrong while rushing and under-pressure).
Show your DJ where you’d like music to be, and ask them how many speakers they’ll be bringing.