^10:3 and he gave a loud shout like the roar of a lion. When he shouted, the voices of the seven thunders spoke.

and he gave a loud shout like the roar of a lion.

This is the strong angel crying out. Angels seldom speak to humans, but this one, perhaps Jesus' bodyguard, cries out as a LION.

This may be fulfilling the prophesy in Isaiah 31:4-7. Note the vengeance as a LION comes right after idolatry even as it does here. See Revelation 9:20.

Hosea 11:10-12 says the 'roar of the LION' will cause 'his sons to return trembling from the west' (Egypt, Assyria, etc.). The Jews and believing Gentiles will return and follow God.

When he shouted, the voices of the seven thunders spoke.

It is not clear what these seven thunder are. John is told to "SEAL UP" what they said. It is NOT for us to know.

We can speculate that

• they were a series of judgments against nature (Psalm 29:3-11 the curse of sins causes flood and subsequent rainbow, (verse 1). All of nature, too, must be released from the curse, Romans 8:18-25.
• it may be the pending SEVEN bowls announcement (verse 7) and it is too horrible. John will explain it as he can later (Revelation 16:1)