Hebrews likely was originally a synagogue address, but 2 Peter 3:15 (And account that the longsuffering of our Lord is salvation; even as our beloved brother Paul also according to the wisdom given unto him hath written unto you) indicates Paul was the author (which Trent also said), yet the style is very different than seen in all of (passionate) Paul's letters (including always identifying himself as the author, and can indicate that the polished eloquence of it was thru the instrumentation of another.
Which means that once again, it is the written version that God chose to preserve, and that this best conveys what God wanted to reveal to man as the pure word of God.