How to redirect sound to loudspeaker from arbitrary app

My Samsung Galaxy SIII comes with a front-speaker and a back-loudspeaker. From some applications, such as the Phone app, it is possible to press a software button during a call to redirect the sound to the back-loudspeaker (so that more than one person can hear what the person on the other side of the phone call is saying). However, I couldn't find the same functionality on the Skype app (or any other videoconferencing or other app). I would like to know whether there is a system setting somewhere or custom application such that whatever app is running in the foreground, the sound it produces can be somehow redirected to the back-loudspeaker from some external software, so that this can work no matter what app (Phone or other) is running. Thanks.