slackbuilds/accessibility/mbrola/README

MBROLA is a speech synthesizer based on the concatenation of
diphones. It takes a list of phonemes as input, together with prosodic
information (duration of phonemes and a piecewise linear description
of pitch), and produces speech samples on 16 bits (linear), at the
sampling frequency of the diphone database. It is therefore NOT a
Text-To-Speech (TTS) synthesizer. 

Mbrola is used in conjunction with a TTS synthesizer. This script
builds the standalone binary for use with a TTS such as espeak-ng,
included with Slackware.

You will need mbrola in order to utilize the mbrola voices included
with espeak-ng.