So far I've dug around in iMovie and Descript and Audacity and ffmpeg and fiddled with feature called things like "loudnorm" and "normalize" and "compressor" without success, but I have no idea what I'm doing so it may just be I haven't figured out how to apply whichever of those is the right option
@simon I've done this manually with EQ/volume/compressor but it's annoyingly fiddly to do well, I'd try something like https://auphonic.com as lazy attempt first