Salmon is one of those words, like perfect, where an etymological spelling has confused folks (no l there either). When it was borrowed into Middle English via Norman it was spelled and pronounced without the l: samoun. Later, people who knew some Latin realized that there should be an l in it (i.e., s;a,o, salmonis), and after that it's a hope, skip, and a jump to pronouncing the l. Likewise, perfect came into English as parfit, but after some etymologizing spellifiers got a hold of that one, we ended up, today, spelling and saying perfect. The sound of an l after a vowel and before another consonant is pronounced with secondary articulation (velarized), the so-called dark l (link).

Ceci n'est pas un seing.