@media speech {

    .spell {
        -epub-speak-as: spell-out
    }

}

As I noted earlier, reading systems will typically have their own defaults, and separating your aural instructions will allow them to be ignored and/or turned off on systems where they’re unwanted.

For completeness, you should also indicate the correct media type for the style sheet when linking from your content document:

<link rel="stylesheet" media="speech" href="synth.css" />

And that covers the full range of synthetic speech enhancements. You now have a whole arsenal at your disposal to create high-quality synthetic speech.