The absolute easiest way is to use Propellerhead Software's ReCycle, which can cut natively at 1/16th note intervals, and then you can export each slice for re-importing into SONAR. You can fool Recycle into thinking the tempo is such that you can slice at 1/8th note intervals.
But, although SONAR doesn't have an automated way to do this there's a
really sneaky method that's pretty fast if you're using loops, or if you're not, using clips that fit 2 - 4 - 8 - 16 bars. It actually might be faster than doing it in ReCycle, and you have your choice of rhythmic values.
But first, to make this as fast as possible, go Preferences > Customization > Editing. Under Clips, for "Selection After Single Split," choose "Both Portions." This really speeds things up because when you split the clip in step 5 below, you won't have to re-select the clip to do the next split. You'll see what I mean when you do this.
1. Set the tempo to the clip's native tempo if it's not a Groove Clip.
2. Open the loop in the loop construction window and if necessary, check "Loop On/Off" to turn on Looping.
3. Set the Slice Value drop-down menu to 1/8 Note and Threshold all the way left to 0%. There is now a slice on exactly every 1/8th note.
4. In the Track view, select the clip, place the Now time at the beginning of the clip, then hit Tab to step through the slices. Each time you hit a slice, type "s" to split the clip. Alt+Tab steps backwards through the slices.
This may sound complicated in theory; in practice, once you've added the transient markers in the loop construction window, the actual tab/split process for a clip takes only a few seconds and it's precise.
There are variations on this theme and it sounds like I should do a "long form" version for "Friday's Tip of the Week," but this should hold you over until then.