The appeal to heaven is likely to mention the direction from which the voice came and perhaps also confirmation that it came from God. In the New Testament heaven is often used as a circumlocution for God (see Matthew 4:17; Luke 15:18, Luke 15:21; Matthew 21:25; John 3:27). Simply put, God himself is the source of the testimony that the eyewitnesses proclaim.1
18 we ourselves heard this very voice borne from heaven, for we were with him on the holy mountain.