converting data sent into audio signal using microcontroller that suits the application
You will need to implement the following at a minimum of you want to achieve this with a micro

●Sound files, of the recorded words you need the system to say during announcements.

●External storage to hold them. 

●A software algorithm to select the correct files and the correct order

●software interfacing between data input, storage, and audio output. 

●Some kind of dac

●some kind of amplifier

●Some kind of speaker

There are things you may want to add, for example file compression to reduce the size of the storage required. Ambient noise sensors to adjust the volume to a sensible level. 

If your micro has a built in DAC, you could boost it with an opamp and feed it into an external PA. Alternatively, you may want to use audio specific ics to get better sound quality and an on board power amp.

What is the scope of this project? What other requirements do you have?