I need to know how "Calm" is exactly pronounced (whether the L is silent or not). And I need a good reference as an evidence.
To understand the potential for variation, I recommend that you start out saying "caw". During this, your tongue will naturally stay at or near the bottom of your mouth.
Now say "caw-m". Your tongue stays at the bottom of your mouth as your lips close for the "mm" sound.
Now say "call" ("caw - ll"). To make the L sound your tongue has to move from the bottom of your mouth to touching the roof of your mouth, just behind your upper front teeth.
Now move to "caw - l - m" (as three connected but distinct, slow sounds). Moving from the L sound to the M sound,your lips close and your tongue moves back to the center of your mouth.
Finally, start speeding up the pronunciation, trying to make the L and M parts closer together and less distinct. At some point you will notice that you don't have to actually touch your tongue to the roof of your mouth; just having it in the middle of the mouth produces a "semi-L" that is distinct from the "no-L" sound when your tongue stays low, and this is what is indicated with the /kȯ(l)m/ pronunciation option.
So, depending on how quickly and how carefully you are trying to say it, you may end up almost anywhere in the spectrum of "no-L" to "full-L" pronunciation.
Variant pronunciations, including sometimes highly questionable variants (eg. "Nook-yuh-lur" as used by George W. Bush), are a fact of English language. While some variants should be avoided in particular contexts, when usage of a variant pronunciation becomes sufficiently common, it can find its way into the dictionaries just as the variants of "nuclear" have.
This youtube video by Emily Brewster, associate editor at Merriam-Webster, is insightful: http://m.youtube.com/?#/watch?v=8nYmWt1J4Lg
The pronunciations of "calm" in the Merriam-Webster dictionary include a variant that does not include the "L" sound and, indeed, pronunciations of this word in certain parts of the United States appear to lack the L sound altogether.
Over the last few years there has been a movement to pronounce certain words "correctly" based on how they are spelled. For instance, February had been traditionally pronounced "Feb-u-ary" because it rhymes better with January. However, there has been a movement in media to say "Feb-ru-ary" whenever possible.
Unfortunately, this movement has caused people in news and television to start hyper-correcting, saying all manner of words that have silent in their entirety. The argument many people make with "calm" is "It has an L!" Well, so does "walk" and there is NO alternate pronunciation of that word.
The answer is the year 1066 and 2000 years of invasions, occupations, and political complexities that go far beyond "correctness." "Calm" does, in fact, have a silent L because of 1066; however, in some regions it has a lightly pronounced L. Why? Because that's how language works naturally over time.
The funny thing is that, even though I know this distinction is the same as "pecan" bring pronounced either "pee-can" or "pik-ahn," I cannot escape the fact that when people pronounce the L in "calm" I feel like slapping them and saying "Please don't do that."