The thing I'd like to know about this mode is: How do you actually use it?
The Adlib Programming Guide mentions it, then promptly dismisses it saying there are better-quality methods of speech synthesis available. The register map in that document gives which bit in which register to set to enable it, but that's it, and the datasheet for the OPL2 isn't any more informative.
Maybe you're just expected to play around until you get it working? Unfortunately, the closest I can get to a system with an OPL2 in it is DOSBox, and I don't really expect the developers to have been perfectionists enough to implement a virtually-unused feature when they had more-pressing problems to concern themselves with.
Ah, well, I suppose I can always try my hand at implementing the sinewave speech synthesis technique in PureData, just to see if I understand it properly. That could be a fun afternoon.