i don't know about tone of voice, mine is always flat, but i've found that most people never smile when they have something really serious they want to say.
when i smile and say something to someone, they will not believe i'm sincere.
maybe you could try smiling when you want to convey a joke. it's worked with me.
of course, it depends on the person you're talking to as well. many people can't pick up on jokes, or if they're not very close to you they could take offense, etc.
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