The writing of the golden Papa speaks with a tongue of trumpet for itself.

my guess would be loudly, at the top of his voice or clearly ?!

Thanks Emotion: smile
Probably, but the sentence suffers badly from various structural problems.
A better way of phrasing the original, in my opinion, would be:
"The writing of the Golden Papa speaks for itself, with the tongue of a trumpet."
Note: 'the' tongue rather than 'a' tongue. We are here supposed to think of the trumpet as some kind of animal. An animal has only one tongue.

Speaking 'with the tongue of a trumpet' means, I suggest, speaking clearly and musically.
Is the Golden Papa a poet or philosopher, perhaps?