I haven't tried this myself, but espeakup is not included from the
beginning in Slackware and has to be installed from a third party source.
Usually when it comes to third party packages,
slackbuilds.org is the
best way to build and install third party Slackware packages.
Espeakup is at
slackbuilds.org at
https://slackbuilds.org/repository/15.0/
accessibility/espeakup/?search=espeakup
But to get that, you will first need to perform a full Slackware install
and then download the espeakup slackbuild and source file.
regards Henrik