Here is the list of phonemes, you can try to use them instead of text. You can also play with the intonation in the phonemes.
Quoting original text by Webbot (Clive Webster):
If you are programatically calling the speak method, or your text starts with a '*' character then
the following text is made up of phonemes with optional pitch numbers.
If you are just 'playing' then it is very likely that you may get an error message - because you have
used a phoneme that is not in the following list.
The valid phonemes are:-
AY as in 'pale'
AE as in 'black'
AA as in 'car'
AI as in 'fair'
EE as in 'meet'
EH as in 'get'
ER as in 'perk'
IY as in 'site'
IX as in 'sit'
IH as in 'sit!!'
OW as in 'coat'
O as in 'cot'
UX as in 'coot'
OY as in 'voice'
AW as in 'now'
AO as in 'door'
OH as in 'won'
UW as in 'you'
/U as in 'put'
UH as in 'wood'
AH as in 'up'
B as in 'bat'
D as in 'dab'
F as in 'fat'
G as in 'gap'
/H as in 'hat'
J as in 'jab'
K as in 'cat'
L as in 'lag'
M as in 'mat'
N as in 'nap'
P as in 'pat'
R as in 'rat'
S as in 'sat'
T as in 'tap'
V as in 'vat'
W as in 'wag'
Y as in 'yap'
Z as in 'zap'
CH as in 'chair'
DH as in 'this'
SH as in 'share'
TH as in 'thick'
ZH as in 'azure'
CT as in 'fact'
DR as in 'dragon'
DUX as in 'duke'
NX as in 'sing'
TR as in 'track'
When using phonemes you can optionally append a number from 1 to 8 to change the pitch of the phoneme.
So to say 'Welcome everyone' using phonemes we could use 'WEH4LKAHM EH3VREEWON'
Since I have no idea how this stuff works, I just converted it into a library, I can't improve it further. If anyone can do a better job, please do and post the code for others to use, as I did, with the permission of the original developer. Also, he said he did his best at it and can't be convinced to spend more time in this matter. He included his code to his WebbotLib library for building robots with AVR controllers.