convert txt to mp3 directly in my Mac

Outside of iTunes, Apple does not bundle an .mp3 converter with macOS, so there is no means to achieve .mp3 conversion of any of the audio output formats from the Terminal's say command, or Automator's Text to audio file action. One would have to install a third-party tool capable of converting .m4a audio files to .mp3 if still required.

For instance, here is an Automator Service that when you select text, and run this service, it passes the selected text into the first action, where it is saved into the designated file and location. That file is encoded as an .m4a MPEG file, and acquires a sequential extension (Text2VoiceFile001.m4a, etc.) as more of these are generated. One then can play, pause, resume the audio file from the Finder.

Hi Daniel,

How to Record Text to Speech Using Terminal...

https://www.iclarified.com/2922/how-to-record-text-to-speech-using-terminal

So create an Applications -> Automator drag and drop app, and then you can do this again and again without needing the Terminal.

Great advice from BobHarris! :)