> >>> Warning. 4GB limit on wav file almost reached.
>
> and the .wav file stops at 4GB.
> So it does seem to be a problem with the 32-bit version of jack_capture.

I don't think so. Jeanette had the same problem when using jack_rec,
and I can't think of any reason why jack_capture should behave
differently when running on a 32 bit OS. It's far more likely to be
caused by the filesystem, or something else. I've cc-ed
Erik de Castro Lopo, the author of libsndfile. He probably
knows more about what could cause max file sizes of 2.1GB.