First off, "taxi" is not a acronym, so is not written TAXI (it comes from "taximeter cab", and ultimately from Latin taxa=to charge)
Next it is culturally wrong. If get in a taxi in the UK or the USA, the taxi driver doesn't say "Welcome". They probably say "Where to?". It is quite possible that taxi drivers in China do use some kind of greeting that could be translated as "welcome" but this doesn't translate well.
When we do use "welcome" we might say "Welcome to my home" or "Welcome to our school" when greeting a visitor and guest to a place. We also say "You are welcome to sit anywhere" when allowing a guest to do something. Neither sense works well in the context of a taxi.
Personally, I would just turn off the robotic voice, or say something in Chinese, however, if Marketing feel that the taxi has to say something, a phrase like "Thank you for choosing "A1 Taxis". We hope you have a pleasant ride." is suitably anodyne. (Anodyne = inoffensive) You could give some information: "Customers are reminded that smoking is not permitted in the taxi cab"
"You are welcome to take our taxi" is grammatically correct, but it could mean "you may ride in our taxi" (thanks, but do I really need your permission?) or it could mean "You may steal our taxi" (Wow, free car!). I don't think either meaning is intended. You don't need to tell a customer that they can ride in a taxi, they already know that.
If I am in China, I would be grateful if the taxi driver speaks some English (given that "ni-hao" is the limit of my Mandarin) but I don't need the taxi to talk. If you want to have a welcome message only to be polite to English speaking visitors, be aware that we neither want nor need taxis to talk!