As sure as I called the previous PCB layout "final," I came across a solution to a problem I'd seen but not known how to fix: putting the mike input and speaker output into a single cable. (Most applications I can think of for this would have both extended to a single unit, like a headphone with mike, rather than two separate locations.)

Not knowing the standard, I've gone with IN+ on the tip, OUT- on the first ring, OUT+ on the second ring, and IN- on the shield. If I find out the standard for TRRS cables, I'll change it to fit, but that's what it is for now.

To my slight surprise, this also enabled me to move the right-angle headers over to allow a bit more room for them. I might even try to figure out a way to orient them so they're at the edge of the board, rather than pointing across the middle.

All this very nicely frees up a lot of space on the board, so maybe there's some aspect of this that I could add...? (I'm very open to suggestions!)