A:
If you're not at an intermediate level yet, I don't recommend diving too much into every little detail.
Generally speaking,
numbers ending in -teen = stress on teen
numbers ending in -ty = don't stress the -ty
Now, here goes more detail.
If you're saying the number or any word by itself, you'll follow the word-level stress. But there's also sentence level stress. In English, you don't stress every word in a sentence. This is part of what makes English sound the way it does.
For example, we know how to say "part" and "of" but I would rarely say "of" as "of".
It would sound something like "partuhv". Because there's no stress on "of".
If you say every word the way each word is supposed to be pronounced, the greeting would sound like:
How. are. you.
But instead we'd hear something like:
Hawuhr yuh?
In the 9-second audio, if he pronounced every number the way each number is spoken alone, it would've been confusing to listen to. But because some parts are stressed and some are not, it's clear which numbers are grouped together and if the list is over. Stress isn't just making them sound louder or longer, it also includes pitch. And it's probably the changes in his pitch that you're hearing.
The same goes for the newer audio. He stressed the 18 because he wants to emphasize it. It's a long time and he doesn't regret it.
Again, these maybe too much to dive into at the moment. But if this is really something you're interested in, then search terms like English rhythm, intonation, cadence, stress pattern when speaking would help you find more resources about the topic.
Rachel's English on YouTube covers this. But as much as the channel is informative and helpful, I personally think Rachel's isn't something I'd recommend as a starting point for beginners.
I recommend this kind of topic when you're at that point where you just want to polish your speech. Then again, your learning journey and pace is up to you.