channelLayoutMask

Which speaker each channel belongs to, as FFmpeg's native order mask: one bit per speaker.

channels alone cannot answer this. Six channels are 5.1 with side surrounds or 5.1 with back surrounds, and a downmix that guesses wrong sends the surround content to the wrong speakers. Null when the container declared no layout, or declared one no mask can describe (a custom channel order, or ambisonics). A caller that gets null falls back to channels and should say that it did.