Good idea, the use of the sifflet is great we prefer entry 3 with the little character The whistle is a good idea. We like the style, simple and flat, which also sort of reminds speech bubbles, like we use on the site to display tips, i don't know if that was intentional :)
Entry #12 is my favorite, the graphic with the whistle and bubble is great. Could you try to combine the letters "CO" you did on entry #68, with the "play" symbol inside the the O?