The trick to making your prop loud is to get the tip speed up over about 0.8mach.

circumfrence the prop tip is traveling is pi * diameter, or 3.14 * diameter. Multiply by RPM to get inches/min and convert units. Look up speed of sound at your altitude, and compare.

The larger the diameter, the faster the tip speed, the more noise you make at the same RPM.

Of course, when props get going this fast, they also loose efficiency, and that noise is energy that's being "wasted", so the louder the prop, the more energy is going to sound and not into moving air.

If you just want more noise so you can hear your plane better, then the best way is to mess with the muffler, then you get more power and more noise. Depending on the engine and muffler, there is a lot you can do. Many glow engines have baffles in the muffler. OS and Evolution for example. Take the baffle out and you get free power and noise (well, not quite free. You get less backpressure to the tank as well, which may or may not matter depending on your fuel system). On some mufflers, you might be able to drill out the exhaust outlet a little as well.

Take the muffler apart and look for flashing or rough edges as well. I worked over the mufflers for my limited-class .25 combat engines and picked up 3-500rpm just by cleaning out flashing and such in the mufflers. Of course, in this case, the muffler isn't much louder, but you get the idea.

Replacing the muffler works. I have an OS .32 with a Slimline pitts muffler. The outlet area on the slimline is much higher than on the stock muffler, and that little .32 has quite a bark to it now compared to the stock noise. (I also turn it at a low RPM with a big prop, so it really confuses people. It doesn't have anything like the typical small two-stroke whine, it's more like a mini-gasser bark).

If you want a LOT of noise, see if you can find one of the old style non-muffled tuned pipes. They make a whole new category of "loud" possible.

Have fun, and I hope you don't fly near me