OpenAI will absolutely add voice and my guess is that their voice support will rival anything on the market because they will train the voice model alongside the text and image models. This is likely months away if not weeks away.
Obviously just my $0.02:
I'd start building for the enterprise right now. Visualize a future where there are several multimodal AGIs that work with voice, images, and text. Be the enterprise voice layer for all of them. Build your moat there.
I don't think there will be any demand for a self-hosted voice model with a SaaS LLM though. So that only works if they are going to train an LLM from scratch (or take the legal risk of using LLaMA).
Obviously just my $0.02:
I'd start building for the enterprise right now. Visualize a future where there are several multimodal AGIs that work with voice, images, and text. Be the enterprise voice layer for all of them. Build your moat there.