Dear Tom,

There are many good tips on just what you're asking on www.chiffandfipple.com

If those fail, and, if you absolutely positively cannot loosen the plastic mouthpiece, go out and spend a couple bucks on a Susato, which is a plastic whistle that sounds very nice and has a tuning slide. These are very inexpensive and are worth the price.

Very best of luck to you,

Bob Pegritz