Orca is a free, open source, flexible, extensible, and powerful assistive technology for people with visual impairments. Using various combinations of speech synthesis and Braille, Orca helps provide access to applications and toolkits that support the assistive technology service provider interface (AT-SPI), e.g. the GNOME desktop and Unity. The development of Orca has been led by Joanmarie Diggs at Igalia, with contributions from many community members.
Alt+Super+S is a shortcut to enable/disable Orca.
(Super is either the Windows Key or Command Key depending on your hardware)
If you press that shortcut it should stop the text to speech functionality. You may have pressed that combination together by mistake.