The iGO_World version is not included among the supported programs in pongo skin. This can still work, but there is no guarantee. It did not turn out to be TTS or traditional sound.
skin settings/warnings/speed limit/Warn When Speeding - three options: Audio and Visual / Visual / Disabled

If only the speech disturbances, a little lower in the menu it can be adjusted

Warning Type or Alert Type: Voice and alert sound / Only voice / Only alert sound
Under
Announce speed limit

If they do not work, you should go to the latest pongo ux-skin package, and select a supported nextgen version (iGO_Avic, iGO_Basar, iGO_Israel, iGO_Pal)