Yelling, shrieking, and so forth, is rather difficult to do. First there’s the fact that you don’t want to damage the ears of your listeners. And secondly there’s the technical aspects of the hardware – the microphone is rather sensitive.
When the microphone is set to comfortably pick up a normal talking voice from a foot away, suddenly switching to a shout will blow it out entirely, and the audio will sound like a garbled mess in play back. One possible solution is to back away from the microphone, but that changes the acoustics noticeably. More common is the “stage shout” where you try to pitch your voice like you’re shouting but without changing your volume. This is demonstrated by Daphne and Neville in this episode, and while it’s not ideal, it’s the only practical approach for the casual participant. (This is not to criticize, I don’t expect anyone to blow $100 on a mic just for a few lines. I’m immensely grateful to everyone that’s contributed voice work!)
The best solution I’ve found so far is to turn the gain way down during the loud bits, then turn it back up again when returning to normal speech. This allows you to get the tone and inflection of an actual yell while getting decent audio quality and also preserving the eardrums of listeners. It’s a bit of a hassle, and it requires a slightly more expensive set up, but it’s worth it. I didn’t do it for Draco’s “Somnium, curse you!” and “Luminos!”, as I hadn’t thought he’d be yelling loud enough to blowing out the mic, and you can tell. Harry’s “Stupify”s aren’t perfect, but they were quite a bit louder and don’t fuzz out as much.