In Renoise 2 beta 2, the sample in use in Renoise instruments would “select” itself, so it was pretty easy to find and edit. what is that function called, and how do I turn it back on?
thanx
First of all, you can download beta 3 already 
the function you describe is áuto-capture nearest instrument.
in the GUI search for Capture Instr.
you can even assign a key shortcut for this function.
If I understand you, its located in Instrument Editor, check/uncheck Autoselect Played Splits.
now it works again 
It only autoselect when playing back the instrument via MIDI/Keyboard. It wont autoselect when playing back something in the pattern.