The Seven Thunders are definitely heard before the seventh angel sounds. Even within the sequence of chapter 10 (a inset), the Seven Thunders will occur before Revelation 11:15. The Two Witnesses do not begin preaching until chapter 11, so the Seven Thunders sound before the Two Witnesses begin to preach.

The events of chapters 10 and 11 must occur absolutely before the seventh trumpet sounds—they even begin and end before the fifth seal, well before the seventh trumpet, which is the last part of the seventh seal. This helps us understand what the Seven Thunders are.

Chapter 11 begins before the Tribulation and the Day of the Lord. Given that Chapter 10 is part of the same context, the Seven Thunders conclude before John, the type of the Two Witnesses "must prophesy again." What we see at the beginning of chapter 10 is God's message—the gospel of the Kingdom of God—being given to mankind in seven sequential blasts!

If the Thunders had pealed all at once, John would not have been able to count them, but he distinctly hears seven of them. The events of chapter 10 blend right into Revelation 2-3: The Seven Thunders are the messages of the Seven Churches!

John was told not to write them, not because their message was a secret, but because it is already written—in Matthew, Mark, Luke, John, etc. To give it again would have been redundant. The giving of the message must be finished before John "must prophesy again"—that is, before the Two Witnesses preach during the Tribulation and the Day of the Lord.