You can turn speech off permanently in the Manage-Database-User window. This will stop messages from being spoken by default on login.
If you want to stop the current reading of a Message without actually turning off speech then click the picture of the crossed ear on the right side of the window.
If you want to turn speech on or off for the rest of this login session, but not off permanently then right-click on the Messages window and choose the option to Turn Speech off/on.