We've talked about the onset, or attack of a musical note. Another element to consider is its staying power. A piano note decays fast, so that a whole note held over four beats will be very much softer on beat four than on beat one (depending on the tempo, of course.) An organ whole note maintains its original volume over the four beats (more or less). A saxophone note might even get louder.
Morton Feldman wanted to get away from the excessive emphasis on articulation and attack, and the way that instruments became parodies or "stencils" of themselves. He tended to want his music played "as softly as possibly."
I'm not sure what this is a metaphor for. I'm sure it's a metaphor for something. Possibly prose itself. You have to know what kind of instrument you are playing. A steel drum or piano will have more repeated notes to compensate for the quick decay of sound. If you think of your instrument as woodwind or brass, your metaphor will be breath. If you are playing a string instrument while writing, you will need to work out the bowing.