I've not ever heard of putting any type of "microphone" near a bird so that you could hear them better, lol, it's usually the opposite problem where the bird is way too loud and the owner wants to quiet them down...But I suppose if you wanted to you could rig a microphone next to his cage, lol...
As far as them getting louder with age, I've not ever noticed that in 32 years of owning Budgies. My first Budgie was a male and he was absolutely the best talker of any of them that I've ever owned, most-likely because he was an only-Budgie and had only myself and my mom to listen to, instead of other birds. He was a great talker, in-fact, male Budgies are one of the best talking parrots overall, with the World Record for the bird who says the most words being Disco, a male green-pied Budgie. So I'm not surprised that your guy is talking at 9 months old.
Budgies are small birds, and while they can be extremely loud when they are singing/whistling/squawking etc., they probably aren't the loudest when it comes to them actually "talking" in human words. Again, I've not ever noticed them to be too quiet to hear speak, but definitely quieter than say my Green Cheek Conure or my Senegal Parrot, and certainly a lot quieter than my mom's African Grey. I don't think that they get "louder" as they get older, but rather that they learn more and more words, phrases, and sounds as they get older. So I wouldn't think that he's going to get much louder than he is at 9 months-old, as he's not going to get much larger in size than he is now, maybe a bit, but at 9 months-old an American Budgie is just about fully-grown as far as their overall body-size. But he will most-likely keep learning more and more words as he gets older...
