This site gives instructions for knitting bells, - too much work for me - but perhaps another project for felted sweaters. Whit's Knits: Felted Bell Ornaments - The Purl Bee - Knitting Crochet Sewing Embroidery Crafts Patterns and Ideas!
These are not mine, but I make a different type of door bells. I call my Universal Charm door bells, "A-door-a-bells." I used to hang these on my doors like a merchant door bell that sounds whenever someone opens a shop door to come in or leave.