Question: In the current draft of my book, the evil dictator whose body is encased in a silver alloy talks basically by allowing magic to move to seep into the silver shell, dispelling it and causing vibrations.

This voice sounds alien, imperial, and metallic. In order to represent this, I'm currently using a different font. (Specifically Kino MT.)

I do something similar with a group that constantly wear helmets that filter out certain wavelengths of sound, and use the 'Cracked' font.

They also sometimes speak in an imperial dialect which I represent with angle brackets.

Is this just going to reek of tacky amateurishness and such?

Answer: 
This is generally inadvisable (which is not to say that it is not done sometimes). The reason it is inadvisable is that every artform has its palette, its set of devices and conventions by which it tells its story. Mastering any art form is about learning how to tell the story within the confines of that palette. 

One of the limits of the prose palette is that is does not support sound effects. You can describe sounds with words, but you cannot reproduce them. 

One of the limits of the movie palette is that you cannot describe sounds, you can only make them. This means that all sounds in a movie are presented literally, and that it is impractical to present sounds that are injurious or painful to humans. (Movies have to fake this by using non-painful sounds and having the actors writhe dramatically -- something every third Star Trek episode seemed to indulge in.) Prose, by contrast can describe sound by metaphor, suggesting a far richer experience than a mere speaker can produce. 

So, you treat sound differently in prose and on video. 

What you are proposing is to represent sound through a font change. This is not part of the common palette of prose, so reader will not know what to make of it. This will tend to pull them out of the story world to ponder the meaning of your typography. But more importantly, there are other, richer, and more conventional ways to handle this in prose. 

You can describe the nature of the sound the first time it is heard, but if you really want to make a voice distinctive in prose, your best tools are distinct diction and distinct message (what is said, and the words chosen). 

Don't try to extend the palette of prose; try to become a master of the whole of the existing palette. It is more than adequate for any story you might wish to tell.
In my humble opinion, don't.

You'd have to explain what the different fonts mean. You can't expect the reader to guess. Perhaps that could be done smoothly. If I was reading a story I'd find it rather jarring, tearing me out of immersion in the story, if there was a note in the middle of the story that said, "When I use this font it means ..." That's breaking the fourth wall. Okay, maybe you could slip it in more subtly. Like the first time you use it you say, "Then the dictator spoke by blah blah" and then give the text in this different font and the user should get it.

How many different fonts are you thinking of using, and how distinctive are they? Readers will have to be able to instantly distinguish the different fonts and remember what they mean. You may say, "Well obviously this is Garamond and that's Times Roman", but that may not be so obvious to a reader who isn't much interested in fonts.

I'd guess publishers would not appreciate the extra expense for typesetting and proof-reading.

I've read a few stories where they used different fonts for different types of speech, or a different font for voice versus telepathy, or humans versus robots, or whatever. Generally I've just found it gimmicky and distracting.