I'm now learning this for a concert I'm doing in June. It doesn't seem to be played much now; Narciso Yepes had it in his repertoire. The Cancion is short but very beautiful, with some rather strange chords. There is one recurring D minor chord which everyone finds really hard as it involves the 4th finger holding the D on the 5th string while playing the F A and D on the 4th 3rd and 2nd strings using fingers 2, 1 and 3 respectively. You will find that the fourth finger blocks the other notes, or if they sound you cannot sustain the D. My solution is to adjust the fingering so that the previous chord uses the 2nd finger on the 2nd string playing the F; you then do a sort of "Bearclaw" chord with finger 2 on the D, finger 1 on the A, finger 3 on the F and 4 as before. With a little practice it is quite smooth and you can hold all the notes!