Due to the finite speed of sound, the explosion would have had to occur approximately 20 seconds before they heard it. So if Voldemort's death was coincident with the explosion it would had to have happened about 20 seconds before Harry said it did.

She'd just about decided that this had to all be a prank in unbelievably poor taste, when a distant but sharp CRACK filled the air. [...] "It worked," Harry Potter gasped aloud, "she got him, he's gone." [...] "I think it's in that direction." Harry Potter pointed in the rough direction the CRACK had come from, "I'm not sure how far. The sound from there took twenty seconds to get here, so maybe two minutes on a broomstick -"