. . . calm . . .calm . . . [audio source]
• (UK) IPA: /kɑːm/, X-SAMPA: /kA:m/ • (US) IPA: /kɑm/, /kɑlm/, X-SAMPA: /kAhm/, /kAlm/ [wiktionary.org]
The first calm seems to be [kɑlm], and the latter [kɑːm]. Do I hear right or both sound [kɑlm]?
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This article has an extended list of words with silent and pronounced "l".
For a language learner, the simplest rule is remembering some most commonly used words that do have silent "l":
Pronouncing the rest of the words with "l" articulated is not necessarily grammatical, but certainly more accepted/understood by native speakers.
Also, see this question on ELU for more details.
Update: as @tchrist noticed, words with -alm may be subject of variations.