If we're abandoning accurate reproduction of sound and just making up anything that sounds plausible, there's already a far more efficient codec: plain text.

Assuming 150wpm and an average 2 bytes per word (with lossless compression), we get about 5bps, which makes 2400bps look much less impressive. Add some markup for prosody and it will still be much lower.

This codec also has the great advantage that you can turn off the speech synthesis and just read it, which is much more convenient than listening to a linear sound file.