Wideners come in three basic flavors (some combine 2 or more of the 3 techniques). Some of them are dangerous. You'll need to find out what method(s) a given widener plugin employs and be aware that some methods can be destructive.
Widening is based on the way our brains process dissimilar input from each ear. It's how we localize sounds in nature, how we evolved to know which direction a twig snap came from, in order to hunt prey or to avoid being prey. Audio engineers who understand that mechanism are able to trick the brain of the listener into perceiving three-dimensional sound information that isn't really 3D at all.
Whenever we hear the same exact sound in both ears - no phase, frequency or time differences - we then interpret that as a pinpoint sound coming from directly in front of us with no sense of direction or width. But we enjoy the immersive experience of being enveloped by sound, as we are in the natural world. But in order to perceive sound as being three-dimensional, each ear needs to hear something a little different from the other. Stereophonic perception is all about making the left and right channels (plus maybe front and rear channels in surround sound)
different from one another.
One of the ways our brains perceive location is through small phase differences between left and right. If a sound is coming from the left, it will take about half a millisecond longer to reach your right ear. We are actually able to sense that tiny phase shift. (At least, within the upper half of the frequency spectrum. Low frequencies' wavelengths are too long). So one method for artificially introducing a sense of width is to insert a time delay between left and right channels. That time delay will usually be short, typically under 5-6 milliseconds. More extreme effects can be implemented with delays up to around 20 ms. Metal music uses this technique a lot for that wide rhythm guitar sound so essential to the genre.
This method is dangerous, because the effect only works when there is adequate L/R separation and the two sides are equal in volume. Works great on headphones and when you're seated in the ideal mix position forming an equilateral triangle with your studio monitors. But play the sound back in mono, or listen from the next room where the stereo channels have been mixed acoustically to mono, and the width illusion disappears. Worse, it's replaced by comb filtering that makes your music sound thin and hollow.
Fortunately, there are other methods that aren't as dangerous. The point is to create (or exaggerate) differences between the two channels. One method is to use complementary filters to generate L/R differences. This is completely safe, because when you combine the different channels you get the original, unfiltered tone. So we say that it's mono-compatible. It can also be used to fake stereo from a mono source. There are a number of plugins available that use this technique, such as Meldaproduction's
MStereoSpread. You can also do it manually by cloning a mono track and then applying graphic equalizers to each track, set to opposite settings.
The third approach is to just accentuate whatever L/R differences already exist, rather than creating artificial differences. This, I suspect, is what Widemouth does (I haven't used it so I can't be certain). It's known as Mid/Side processing. The idea is that you separate the parts of the music that are
common to both sides from the parts that are
different, and adjust them (EQ and/or volume) independently. It's actually simple math and not as magical as it sounds. The Mid component is everything that's the same between left and right, and the Side component is everything that's different. Turn up the Side, and whatever width is already in there will be accentuated. Boosting the high frequencies in the Side (while optionally dipping highs in the Mid component) also exaggerates width because we're more sensitive to differences in the high frequencies.
Some equalizers (e.g. FabFilter
Pro-Q) let you apply filters to just the Mid or Side components. Some compressors (e.g. FabFilter
Pro-C) let you apply different amounts of compression to Mid and Side components. Some plugins (e.g. Voxengo's free
MSED) merely let you adjust the volume of Mid and Side components independently.
All can enhance width - usually without endangering mono compatibility. However,
this method only works if there are already L/R differences. It will not make a mono track sound stereo. So before you can even think about applying this type of widener, you want to first create as much width as you can using panning.
SONAR/Cakewalk comes with a great stereo manipulation tool called Channel Tools. I use it primarily for panning stereo tracks, but it can also apply multiple techniques for stereo widening, including time delays and Mid/Side volume.
Sorry about the long reply. Sometimes my fingers have a mind of their own. To answer the original question, I have not used Widemouth. I'd say try it out and see what happens. When you've got it sounding good, click the interleave button on your master bus to make your whole mix mono. If everything suddenly sounds like crap, bypass the widener plugin to see if it's the culprit.