If you want to play only 1 second, then make the note only one second long. 
If the note has to be more than 1 second, then set the volume to zero after 1
second (or if the last sample is zero, set the frequency to zero after one
second).  If you want to loop over the one second, then you need to have the
file edited.  Or load the file into a table and use tablei to read it and loop
at the right point.  As for finding loop points that sound good, this is not a
task for which Csound is well suited.  Use a sound editor instead.
-- 
Mike Berry
mikeb@nmol.com
http://www.nmol.com/users/mikeb