If you load Firebird's Booty with the Currah MicroSpeech plugged in, you get an entirely different game?
The game loads as normal, but once you press the key to start playing, something odd happens. For starters, your cabin-boy doesn't appear when the first cabin is displayed and neither do the pirates! Then the new game begins ...
It involves swimming in the sea in a wetsuit, collecting small fish and avoiding any marine life. All the time you air supply is running down. When your air starts to get low, just resurface between ships and stock up with more oxygen. During this under-sea escapade, sailing ships like the one on the loading screen float past.

The keys are the same as those for Booty, but the Open/Hold key has no effect.

Enter the following pokes when the start screen appears to access the hidden game :-

52796,32

52798,58

Btw. You will need an emulator that has Multiface 128 included

eg. ZX80 v 4.00 has this built in

Get it here