I just would like to understand what is going on...

The versions you named don't seem very exotic, and without being able to reproduce it, I can't tell you more. Is qx(ffmpg -f s16le -i out.raw 2>&1) the actual command you're running (is "ffmpg" a typo)? Because if you're doing any other redirections there, the order makes a difference. Otherwise, I'd suggest debugging with strace and perhaps lsof to get some clues why your redirection isn't working.