I'm well aware of a lot of the small updates released by Greybeard and a few others for Red Baron 3D. But I'm always hesitant to post their updates, since their releases are so frequent! But this is definitely something to check out: Greybeard's collection of realistic engine and gun sounds for RB3D!
I haven't tried it out yet myself, but I have unzipped the files and listened to the wav files directly. They sound great!
Download the file here. And here's the install instructions:
Best Sound Set for Red Baron 3D - compiled by Greybeard - 22 december 2008.(via SimHQ forums and vonOben's Flight Sim Mods)
Collects what are in my humble opinion best in-game sounds for realism. If I recall correctly, they are all but one by Sfabert (who kindly allowed me to make of them whichever use I want) and have been derived from actual clips of original WWI machineguns and engines.
First delete any existing *.wav file from your Simpatch folder, then execute the self-extracting one, unzipping into same Simpatch folder. This to get a right combo of "stock" RB3D sounds and modified ones, again the best in my opinion.