Both of the wires are coated and you can't 'cut' the coat off with a knife.
You will have to carefully use your soldering iron until it melts off and solder then sticks to the wire.
Once you have 'tinned' both ends you can then solder them to the speaker.
Scotch brite works quite well for gently stripping the enamel off
For decades I get a little blob of solder on the iron and I keep dipping the end of the wire until the solder finally sticks to it. There's no need for 3rd party stuff.
I';ve never tried just taking it off with heat/solder always assumed it would be imune to it tbh.