Maybe This Will Help:
Clipping
If you don't care about digital clipping or think it sounds "phat," or "warm," or something otherwise unlike complete crap, skip this section. In fact, skip this entire tutorial.
Start at the start, hit play, and stare at the "Audio Out Clipping" LED. If it ever lights up, you know there's a problem. Clipping means that Reason is generating a signal that cannot be properly expressed in a linear, clamped PCM system (which is what your sound card, and also a .wav file, is). By linear, I mean that the string of numbers generated correspond one-to-one with the desired displacement of your speaker at that instant in time. By clamped, I mean that there is a limited range to these numbers.
To borrow a term from the movie, "This is Spinal Tap," digital clipping happens when Reason tells the audio hardware, "play this sound at 11." The audio hardware shrugs and plays it at 10 instead. Thus, what you get is not what you put in, and the result often sounds like wind breaking. The only way to fix it is to bring the signal Reason is generating back within the expressible range. In other words, you need to turn down the volume.
If you notice the clipping is happening at specific parts in the song, say when a particular instrument hits, you may be able to fix the problem entirely by bringing down that particular instrument in the mix. A compressor on that instrument may also be the solution.
If it's happening throughout the song, first try reducing the overall levels in your mixer(s). Turn your monitors or headphones up to compensate for the drop in volume, and then listen again. If you found you've lost some "punch" that the distortion from the clipping was providing, try putting a compressor as a last step before the hardware interface. Start with a midrange ratio value, between 2:1 and 4:1, a high threshold value, and a very low attack and release level. Experiment, and observe how the compressor's gain LEDs react as the audio is being played through. Look out for the distortion that a compressor can create, especially with very low (0-3) attack and release settings. You also have to be careful of introducing undesired "pumping" in the track, with attack and/or release settings that are too high. Also, just because one part of a song sounds perfect with your current compression setting, doesn't mean the entire song does. If this is the case, consider recording changes to the compressor in the main sequencer. Avoid recording changes to the bypass switch, as this can create an audible "tick" when it's switched. Ramp up and down from a zero ratio setting instead.
Be aware that compression reduces the dynamic range (i.e. the difference between the loudest and softest level) of an instrument or mix. Some genres, especially very electronic-based genres are typically heavily compressed, while others much less. Again, experimentation and comparison to professionally recorded songs will help a lot.
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