macOS command line

$ say -v Alex "Hello" -r 200

Save to file

$ say -v Alex -r 200 "Hello" -o test.aiff

Read file

$ say -v Alex -r 200 -f a.txt -o test-a.aiff

Listen

$ open test-a.aiff

Convert to mp3

$ lame -m m test-a.aiff test-a.mp3