From a standalone kernel, try UsingFrontEnd. It requires X server access, so DISPLAY should be set, for example
pi@testpi2lx ~ $ export DISPLAY=:0
pi@testpi2lx ~ $ wolfram 2>/dev/null
Wolfram Language (Raspberry Pi Pilot Release)
Copyright 1988-2015 Wolfram Research
Information & help: wolfram.com/raspi
In[1]:= UsingFrontEnd[Speak["Hello"]]
Mathematica uses eSpeak for TTS.