If you can push a ball gag out, it isn't on tight enough. Also, I've noticed that bigger ball gags seem to muffle me a bit more than smaller ones.
I've used a lot of gags, and none of them have ever made me completely silent, although they do take the octives down a bit.
You can also try bondage tape. It's not sticky, but it stays on surprisingly well. You can even re-use it. ^^