Audio protocol from computer to soundbox?

Is there any information known about how the NeXT computer communicates to the Soundbox?

I don't think the protocol is properly documented. However, there's an FPGA implementation of the Soundbox ASIC (https://github.com/novi/nextasic), see this previous discussion (http://www.nextcomputers.org/forums/index.php?topic=4481.0).

I have shared a document with some folk which describes the ASIC and whatnot in detail.