Consider benchmarking it to the existing Geas/Quest spell (assuming you're either using 3.x or are familiar enough to convert to it).

What you've got in mind sounds very similar, but improved so it's "Anyone who hears my voice". I'd kick up with level 1 or 2 (so, 7th or 8th level) for the Area of Effect increase, and have him use a separate effect to make sure everyone hears his voice... I would base it one something like Ventriloquism (0th level), Spectral Hand (2nd level, allowing the spell to be carried by another medium), and some way to boost one or the other so it can cover the entire continent.