What's the specs of the file you're playing, e.g. resolution, length, filesize, and maybe codec used? And your PC specs? I find this happens if you try to play something too complex (eg hi-res and/or hi-bitrate h264 stuff) on a PC with a processor that's not powerful enough to play the video at full speed :p Basically it can't decode and render the video fast enough, causing it to lag behind the audio. I find softsubs is an especially big performance hit on rendering video, more so when they use those fancy karaoke effects.
Luckily in my case, my laptop can just about manage some of the 1080p softsubbed fansubs out there as long as they don't use those fancy effects, so the problem usually only happens for me during the OP/ED sequences - I just temporarily disable the subtitles during the karaoke then turn them back on afterwards to keep it all sounding in sync :p