Is google speech synthesizer good enough?
Is it worthy to use as a pronunciation reference?
It's very robotic and not smooth but otherwise it is accurate.
Otherwise, most online dictionaries these days provide pronunciation for each word.
WordReference and Linguee both have this feature.
Is it enough accurate in french wovels especially nasal?
Robots typically are correct 95% of the time, especially if you throw sentences at them. If you want 100% accuracy, you have to stick to French natives. :) For instance me in my youtube channel: https://www.youtube.com/channel/UCuEQ46asZFw48r3KtjGkROg Just saying! :)