In those two cases, the English spelling originally did not have an -l-, so there was no such letter to pronounce. The fish was spelled samoun, and the fusible alloy was spelled soudur. In the 18th century, our friends the hypercorrectionists decided that these and other words should show their Latin origins. Salmo was Latin for the fish, and solidus was Latin for the joining agent, so doesn’t it make perfect sense to graft an -l- into the English words? Not really. The spelling was forcibly changed, but the original pronunciation lived on.
The silent L is often followed by one of 4 letters: D as in could, should, would, and solder; F as in behalf, calf and half; K as in balk, caulk, chalk, stalk, talk, and walk; and M as in balm, calm, embalm, napalm, palm, psalm, qualm, and salmon.
The 18th century meddlers threw in other letters as well as they gave a nod to classical origins. A vivid example is the word dette. It was efficiently phonetic, but they decided that since it came from the Latin debitum, it should have a “b” to acknowledge the origin: debt. Doute was not allowed to stand since in Latin it was dubitum; so it became doubt. And the word subtle reflected the fact that in Latin it was subtilis, so the English sutill had to go.
All this makes about as much sense as bemoaning the fact that humans no longer sport their ancestors’ gills.
Another class of silent letters owes its existence to a quirk of history. In the Middle English period, pronunciation changes began to occur rapidly. Without interference, spelling would have raced to keep up. But in 1474, having set up shop in England, William Caxton printed the first book in English. As he and other printers began to print and sell copies, older versions of spelling were chiseled in stone, and soon there was a serious discrepancy between the way many words were said and the way those words were spelled.
The silent -e- on the end of so many words serves a limited purpose in our day. Some would make more sense as som. But once upon a time (tim), some was pronounced som-eh. So came, done, phone, barge, gone, and a host of other words now carry a largely unneeded letter.
The same thing happened with -kn-. A gentle knight was once a gentleh kanicht. So know, knife, knock, knee, knell, knickers, knit, knob, knot and others of that ilk would have lost that initial -k- if printing hadn’t interfered. And as we just saw with the word knight, the -gh- sequence is often unpronounced, as in ought, taught, drought, dough, and many others.
So we have a dichotomous reality: in some cases, people should have kept their hands off spelling; in other cases, someone should have interfered.
NOTE: reader Bob Hageman points out that today the -E- at the end of a word sometimes designates the preceding vowel as a long vowel, as in name, game, blame, and so on. I stand corrected; that's something I forgot to add. I was singlemindedly thinking about English before the Great Vowel Shift, when short vowels were the norm, and the terminal -E- was pronounced with something close to a schwa sound. On the other hand, today a number of words ending in -E- do not sport long vowels: one, done, gone, none, some, etc.