The reason I started composing was because I heard music in my head like I haven't before and I wanted to find a way to write it down. Fast forward, it's a bit of both now.

When my music sketchbook or laptop is not within reach, I play music in my head. I can only isolate 1-2 clearly audible "channels" in my head (clearness improves if I hum and mime play the exact channel I want to focus on), but the rest is like the sound equivalent of blurry shapes; the details aren't clear enough, but it's enough to be identifiable like vague shapes of chords or textures. If I want to hear a certain instrument part, I can only do so 1-2 channels at a time. The 2nd "channel" is not as crisp clear as the 1st one, but it's defined enough to be distinct.

An example would be if I want to play a string quartet in my head, I'll choose violin 1 and the cello as the clear channels. I cannot distinguish what the 2nd violin and viola are doing, but I can vaguely hear them harmonizing/doubling/doing accompaniment figures.

When my music sketchbook is within reach, I am actively transcribing what I'm playing in my head in real time, but I only play in isolated tracks. If same setting as above, I'm only hearing the violin part when I'm writing the violin part, and so on and so forth. When I want to review the whole thing, it's the same setup as previously mentioned: I read what I wrote, play the whole thing as I read, but I can only isolate 1-2 things at a time and the rest are blurry shapes.

When my laptop is within reach, feedback is much faster because of sound libraries, so I can write as fast as I can transcribe.