Have you checked how your playback devices are configured? The dumbest yet simplest thing is to try to disable everything except the one you want to playback sound from. Here is a screenshot for you in case you don't know how to do this. https://dl.dropboxusercontent.com/u/43450951/scr10001.pngIts unmuted. My friend plays SRB2, and when he's on skype, i can hear the volume from his mic. However, my SRB2 is unmuted, loud, yet it does NOT play any sound for me.
Have you checked how your playback devices are configured? The dumbest yet simplest thing is to try to disable everything except the one you want to playback sound from. Here is a screenshot for you in case you don't know how to do this. https://dl.dropboxusercontent.com/u/43450951/scr10001.png