Considering how important Oration was to Ancient Roman scholars, how is it that we have no texts regarding the correct pronunciation of the Latin they spoke?

Apart from that other thread linked to in, which I talk about various current pronunciation schemes.

Firstly, If you just want to know how we know what Latin sounds like, here is a recent, amusing but accurate youtube video.

But since your question is connected to oration and texts, the premise is wrong: there are texts that speak about this.

Quintilian's Institutio Oratio, apart from covering huge terrains of rhetoric, discusses proper pronunciation. That's because he feels like covering practically everything necessary to raise a child to become an orator. The kinds of educational texts that survive, as for example collated in Dickey's Learning Latin the ancient way, demonstrate the concern to teach second-language learners correctly.

Of course, we might wonder why there isn't a text saying, "hey, this is how to pronounce things correctly!" But, who would such a text target exactly? By analogy, if you wanted to teach "correct" pronunciation of English to someone, who would it be for and how would it be written? Such texts are primarily oriented to (a) second language learners, (b) linguists and grammarians.

Which is where you do find pronunciation discussion among ancient authors. Latin grammarians discuss pronunciation as one of their fields.

You don't find it extensively among rhetorical-education texts because the people reading and writing those know how to pronounce Latin (well) and are training to give speeches.

I can come back later and talk more about ancient Latin grammarians more specifically...

edit:

So, this won't help you unless you already read Latin, but almost all the late antique grammarians spend some time discussing "letters", "syllables", "metre" etc., when discussing the fundamentals of Latin. Full texts of grammarians can be found here. For our purposes, big names include Donatus and his Ars maior, Priscianus and his Institutiones, the most famous grammar text throughout the middle ages, and Charisius with his Ars.

Of course, they often don't discuss what we want them to discuss, because they didn't have a fully-developed, say, International Phonetic Alphabet, to record things in. Just as it's far easier to demonstrate a sound aurally, than write about it, so too there's every reason to think that instruction in proper pronunciation happened verbally, in-person, far more than it did through books.