These projectors, and the cameras made for the instamatic line, are all total junk now. Kodak used a really terrible plastic that disintegrates in a pretty short time. In this case, the teeth on one of the gears of that M70 have all been stripped off. Parts are available here:
http://www.kodakparts.com/KodakM.html
Several members of my family shot super 8 home movies, and they used the instamatic cameras. Within a few short years, all had stripped out their drive gears. The stuff was really just junk from the beginning, rather than the parts now being 30 years old.
If you have some film to play, invest a hundred dollars or so in a nicer sound model with 24/18 fps speed selection. Then you can play everything under the sun. Sound projectors are also typically better built so your irreplaceable film won't be shredded.