Finally got my S6.
Open the video from the stock gallery (has the slow motion icon on it). You can edit the start and end points for slow motion as well as the speed and also keep or remove the sound. Then export the file. Share the exported file. If you open the slow motion video from inside another gallery app, it will just play at normal speed with sound.
This is different from the S5, where the video was saved entirely in slow motion. Now it records at 120fps at 720p and you choose what part slows down and how much. That's the way HTC and Apple handles slowmo video.