as a variant on 'soundtrack'

The latter is usually 'bande originale', but strictly speaking, that really only refers to the MUSIC for the film; here, 'berceau' seems (from all the definitions I've been able to find) to refer to the whole structure of the sound for the movie as a whole — which in more technically literal terms is what we mean by the 'soundtrack' in EN.

The problem here is that this very concept doesn't really have a word for it in technical EN — it is a specifically French way of looking at it.

It's all very well saying that it means the same as 'fond sonore' — but if you then try and translate that as usual as 'background sound', you come up against the very problem that this text is discussing. And you run the risk of its being understood as 'a background sound' rather than 'the sound background' — which is not, however, a valid technical term in this sort of context.

To retain something of the 'artistic' side that is to some extent employed by the figurative use of the term 'berceau', I think 'soundscape' might be a better solution; this is the whole audio environment that is created by the sound designer for a given work and of course includes background music, foreground music and other sound effects, dialogue, etc.

In the world of 'synchronizing rights' (i.e. the dues you have to pay to be allowed to use someone else's music in your film), great importance is attached to whether music is 'foreground' (i.e. IN the film, potentially audible to the characters) or 'background' (i.e. just an addition audible to the audience, but of which the characters are unaware).