If you need to strip the insulation from speaker wire and don't have any tools then just use a sharp knife or razor blade. Make a circular cut to separate a cylinder of insulation and then a lengthwise cut to make it easier to remove it. Try not to expose more copper than is needed (usually about a 1/2 inch is fine).
A sharp knife, some fingernails, and your teeth will likely do the trick barring the wire isn't too fine. As AA said, cut around the casing and then pull the casing away to expose the wires. Twist them together into one relatively uniform piece and you should be good to go. No need to cut back any more than you need, either.