I'm aware this song was meant to be changed to provide Natural AC instead of dodge AC. I've just tested it and at level 9 bard it seems to be providing +1 AC (stacking dodge AC). I've tested this without and with the effects of barkskin and shield. This result is the same - a +1 AC bump. I'm not sure if this effect is higher at level 10 and maybe higher-level bards who frequently use this song might be able to advise. Either way, this song might need to be looked at again to figure out why it's doing this.