The piece was composed for solo piano, so likely the extra instrument that you're "hearing", Frank, is the obbligato or variation in the left hand. The left hand is usually lower in the tessitura (overall pitch) so the sound isn't quite as prominent.
Thank you, sir.
It's a habit of the deaf, or the near-deaf, to use the imagination to fill in gaps of things (usually things spoken), and what I was "getting" from the background seemed a second instrument, almost as if a set of billows breathing in-and-out.
In my perception, it added considerably to the more-prominent music coming from the piano. At any rate, for the first time I could recall, the piano-played music didn't seem "flat;" it had depth.
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Now, for those who are curious about how a deaf person can hear, there's various levels of deafness, being "mild," "moderate," "severe," "profound," and "total."
My own range is mired in the "profound;" a graph of my hearing would look as the wavering lines on an EKG of a dying patient.
.....you may sometimes hear or feel very loud sounds like lorries passing in the street.
"Total deafness" is actually almost unheard-of, because the body is always absorbing sound from the environment. One feels the muddled, tenuous sounds, but has no idea what they are (and so considers them, wrongly but humanly, as being of no consequence).
And the ability to "grasp" it has several variables, including one's state of mental alertness at the time, one's sense of well-being at the time, and probably even things such as air-pressure, humidity, temperature, whatnot.
But nearly all the time, it's not worth even trying to figure it out; it's too hard, like laboring a mountain to bring forth a mouse.
However, for whatever reasons, the other night, the conditions must've been ideal, and so I caught this. If I were to "hear" it again, I probably wouldn't get it so good.
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And one "hears" different qualities at different ranges; for me, plucked strings are usually distinct, while percussion instruments aren't--thus my idea that the harpsichord is the most delightful musical instrument ever contrived by man.
An assessment of course simply because I can "hear" it, at least somewhat.