It can be done easily either using a plugin (like the XMPlay MSN or last.fm plugins), or using external software (by reading the window title). I did the latter many years ago using Visual Basic 6 and it was rather simple. No idea how hard or easy any of that would be with OBS though. Does it have a way of extracting window titles from arbitrary windows? If so, you can look for a window whose class name is "Winamp v1.x" and then read its title.