First, a definition: when I use the word "sound," I'm referring to a particular kind of experience. Snap your fingers and listen. What you hear is a sound. Now, imagine that a tree once fell in a forest when nobody was around. Whether or not it actually produced a sound is a matter for philosophers. But unless you actually heard something just now (which okay maybe you did if you have a good imagination...), I'm not calling that a "sound" for our purposes. Maybe you want to call it an "abstract sound" or a "theoretical sound" or something, but make a clear distinction between that and things you actually hear. So here "sound" refers to a particular kind of experience, not some abstraction.
We say that we "hear a sound." There are two things: hearing and the sound. More generally, we say that we "experience an experience." The word "experience" is both a verb and a noun, reflecting how we think of perception.
But notice that there's no such thing as "hearing" in the abstract -- that is, hearing without a sound. Whenever you hear, you hear a sound. Similarly, there's no such thing as a "sound" in the abstract (recalling our definition above). The category of experience we call "sound" requires hearing for its very definition. Spend some time with this in your own experience. Try to find an instance of sound that isn't heard, or an instance of hearing that doesn't involve a sound. You can't.
This may seem mundane, but it's not. According to the nondual traditions (which exist both in the east and in Abrahamic mystical traditions), the very reason we suffer (i.e., feel anything less than completely satisfied with and fulfilled by existence) is that we mentally split an intrinsically undivided and indivisible reality into two parts: the subject (I, doing the experiencing) and the object (the world being experienced). As a result, we believe ourselves to be fundamentally isolated, like a man on an island whose communication with the outside world is limited to bottles floating back and forth. You may think "I don't feel isolated!", but if you have a sense of being "in here" while the world is "out there," then indeed you do feel that way, whether it consciously bothers you or not.
When we're really engaged, like in the state of flow, that imagined isolation can drop away. But even in flow, unless we have a remarkably high degree of awareness, we can subtly miss out on the wonder of what's occurring. That's why hours pass like seconds in that state. If you lived your life in that kind of state, maybe the next five decades would pass in minutes.
The second difficulty is that we conceive of flow as a particular kind of state, which by definition we can enter but must inevitably leave. Sometimes people describe Zen in this way.
But Zen isn't a particular experience or state. It's a realization: that the world wasn't actually divided until we inserted that woeful division ourselves. In fact, it could never actually be divided at all. That was all just a funny dream we invented, a little game we played with ourselves so that we could have the joy of rediscovering the astonishing and ravishing truth of existence.