There is no such thing as "silent letters"; there are only "outdated spellings".
I'm sure you remember, like I do, learning how to spell using "silent letters". Like the "k-" in "knife", "know", and "knight". Or the "-gh-" in "though", "through", and "thought".
But let's stop for just a second. Who's evil idea was this? Are we putting in random silent letters just to torture young children who are trying to learn how to read? Just to laugh when they try to sound out words with sneaky spellings?
As it turns out, no. These aren't silent letters, but outdated spellings. Outdated spellings are ones that reflect how words used to be pronounced in English. The are spelled that way because they were pronounced that way! It's us moderns that are either 1) saying them wrong or 2) spelling them wrong.
Let's look at a few examples:
"Daughter" has a spelling that shows its Germanic roots—in German, it is spelled (and pronounced) "tochter".
We can very easily see how English (a Germanic language) used to pronounce the now-silent "-gh-", almost just as the German still is pronounced.
The word "knife" was borrowed by the French before the "k-" became silent—it's now the word for "pocket knife", "canif" (pronounced with a hard "c").
American spellings were, in part, an attempt to address some of these issues—which is why we have "draft" replacing "draught", among others. Before spelling standardization, all American and British spellings had their roots in, believe or not, Britain. But when the Americans tended towards "English" spellings that reflected English pronunciations, the Brits reverted to French spellings for French words. Which is how we get pairs like "color" and "colour", and "program" and "programme".
In any case, it's a universal rule that speech naturally changes and evolves. Writing, on the other hand, changes much more slowly. But the more of these outdated spellings that pile up, the harder and harder it gets to learn how to read. Children in English-speaking countries have a more difficult time learning to read than German-speaking children, for example. That's because German is spelled how it is pronounced!
Tibetan has the same issues.
You'll notice that, if you've learned Lhasa or Central Tibetan, there are a plethora of "silent letters". The fact is that these weren't always silent letters.
Just as spoken English has changed over time, leaving us with outdated spellings, Tibetan, too, has changed over time. The Central dialects have "silenced" many of the prefix letters, just as English silenced the "k-" prefix letter.
Not all Tibetic languages are pronounced the same, however. This video is a bilingual, audio version of names of fruit in Chinese and Tibetan—I like it because we can see how the Tibetan is spelled alongside its pronunciation. You'll very clearly hear letters pronounced in this speaker's dialect that wouldn't be pronounced in Central Tibetan!
(On that note, I believe it's an Amdo dialect; if anyone can identify it with a bit more precision, please let me know and I'll add it here).