I'm curious, what is the business justification for funding development of Demucs, if you don't mind me asking? It doesn't seem very related to FB's core business.
Solving problems like audio source separation (eg. Distinguishing multiple speakers in a noisy environment, or picking speech out of a background where music is playing) seems very much in FB's wheelhouse.
The goal of Meta AI Research is to do open research, not necessarily with direct applications at the time we start it. Indeed, the architecture, or the lessons learnt working on it can become useful later for the company, for instance for remote presence with VR, to isolate the main speaker from its environnement ( https://arxiv.org/pdf/2206.15423.pdf ).
Just a guess here, but I wouldn't be surprised if it's used to better spy on your messenger audio conversations. They already listen in and will pick up keywords to populate your FB ad stream.
If I can reconstruct your conversation (through other meta information), without listening to sounds of your voices, have I not listened to your conversation?