Try this, llessard:
'Concrete' means tangible, physical; 'abstract' means conceptual.
I can see, hear or smell any action. I can hear a drumbeat or a sneeze, I can swallow helium (or any of the elements, if I have suicidal tendencies), I can see you taking a walk or having a short run.
You can taste the lemon or the citric acid within, but the sourness is the mental concept of that taste, so 'sourness' is an abstract noun.
You hear the chords which are a physical combination of tones-- chords and tones are both physical constructs-- but whether it is music or not depends on your sensibility. Some would call 'rap' music, but I would not: 'music' is in the mind of the beholder, hence an abstract noun.
Does that help any?