I have found this post from 2016 and although it claims to give an answer, it doesn't, so I'm raising this issue again.
The audio in Film 4 programmes is very low compared to every other channel, however, the audio in adverts on the channel is fine.
It's as if the audio mix is not correct because it seems to be the voices, or what would probably be coming from a centre speaker in a multi-speaker setup, that is low, background music and effects etc are fine, so you can hardly hear people talking unless you increase the volume, but then it is way too loud when music is played which makes the HD channel unwatchable. The difference is obvious when flipping between the SD and HD channel.
This is just using a TV with stereo speakers, connected via HDMI, and every other channel is absolutely fine.