Audio software developers forum.
I think we've got it now. I can't post as much detail as I'd like, because the solution ended up being in a part of the code I'd not been shown. It wasn't our rounding exactly that was the issue, but rather, when the rounding was occurring, the total number of bytes the file was expecting was not adjusted for the rounded value, so the total number of bytes was too great/few. Really could not have done it without you, Steve. A very tired and frustrated engineer (not me) now gets to rest easy.