Realtek is the onboard soundchip that's built into most motherboards today. It could be that your computer is set to playback audio though that and not through your soundblaster. I don't think the realtek chip can handle anything above 44.1 so if the audio you are trying to play back is at 48.1 it would play back at the wrong speed.
Make sure that everywhere in your audio chain you are using the soundblaster. Check in control panel/Sounds and audio devices under the audio tab that it's set to playback using the soundblaster card and not realtek.
