You have a 'Stanley knife'? - common UK name for a generic tool I know of no other name for.
Hold wire against knife with thumb.
Carefully push thumb so wire rotates.
Stop before blade reaches thumb.
Pull end of plastic insulation off.
if it fights you a bit, drag the tip round in a circle, putting pressure on each side in turn.
This has been the trade ...
Greenlee 1161 Adjustable Fiber Jacket Stripper
I used to use these until I learned how to just do it with a utility knife.
If the jacket is loose enough, you can try these.
"Slitting blade [not visible in picture; it's inside] rips outer jacket of NM cable cleanly and quickly"
IDEAL Lil' Ripper Wire Strippers
I've owned "The best wire ...
Being stingy, and not averse to minor personal injury I'd attack the white outer with my trusty pocket knife,
The idea is to cut it most of the way through the white layer and then flex the cable until it tears the rest of the way though.
Cutting while flexing works well too.
For speakers, losing a couple of strands of the wire itself is irrelevant. I know that some of the wire mfrs hawk their "gold plated" wire and make a huge deal about conductivity, but in reality it's all bunk. 16ga wire is rated for 10A, even a typical 350W speaker is going to draw maybe 6 to 7A.
With experience, you can just cut the outer insulation (trick being, not all the way though - you make a weak spot/stress concentration by grooving it, and then pull off the waste, and it breaks where it's been grooved.) You can practice by cutting off less of the outer sheath than you need a few times, so any nicks on the inner insulation would be stripped ...