You most likely have installed the +Speech script, which provides voice notification of events like contacts signing in/out or status changes.
You can check if it is installed, as well as remove it by going to Plus! > Preferences > General (tab) > Scripts (side tab).
Check if "+Speech" is in the list of scripts, if it is then you can remove it by clicking the "Remove" button on the right hand side.
If it isn't there, list which scripts you do have installed, if any, so we can try and find which one is speaking to you 
/ –
