I was never able to get a SetVolume command to appear, even on entirely different machines with entirely different 4.18 projects. If Dan hadn’t been trying so hard to help in this thread I’d be convinced it just wasn’t possible.
In the end, I stumbled through an awkward workaround using a SoundMix, a SoundClass, and a SoundMixClassOverride. SetSoundMixClassOverride has a volume input. I’m convinced it’s all wrong, but I can at least change the volume of video now!