Step 4: Adding the straw
Squash a piece of drinking straw into a flat oval, and experiment with positioning it at the hole whilst blowing. It needs to blow across, yet slightly downwards.
When you have the best position figured out, tack the straw in place with a couple of drops of hot-glue, check again that the position is correct, then add more glue around the straw to hold it in place.
I also trailed a couple of strings of glue over the straw, with the idea that they would keep the straw in the correct oval shape. I don't know if they are actually needed, but they didn't damage anything...
Anyhoo, there you go - the whistle has a pleasant tone, a bit like a small train-whistle, and is not so loud that small children will run screaming from the room, or get in major trouble for playing with it in the house.