Yesterday, my Finnish co-worker lent me a CD of the music of Sibelius, which, unaccountably, I've never heard before. (I suppose I must have heard "Finlandia" on some classical-music radio station without knowing that that's what it was: I mean I've never knowingly listened to Sibelius.) As I was reading over the list of works on the recording, I thought, "Didn't he write the Turangalila symphony? That sounds pretty Finnish!"
No and no. The Turangalila symphony was written by Olivier Messiaen, who's French, and the name isn't Finnish at all: it's Sanskrit.
You know what else is Sanskrit? The word "sandhi". I didn't even know it was an English word, but it is, more or less: it's used by linguists, and if you don't know it either, you will now, and you'll wonder why you never heard of it before.
Languages with alphabets have rules as to which letters can be combined with which other letters, in what order, and in what place in a word. An English word can never start with "ng-", because we need to precede that pairing with a vowel, but that doesn't mean that no language can start a word with that combination--there's a writer named Ngaio Marsh. (In fact, when English speakers pronounce her first name, which is that of a tree in her native New Zealand, they generally jam some sort of vowel sound in there, either at the beginning or in between the two consonants, so strong is our need to have one: "Ing-eye-oh", or "Nuh-guy-oh".)
One of the letter combinations that doesn't fit the English-speaker's mouth very well is "-np-". It does occur fairly frequently: it's easy to tack the prefixes "non-" or "un-" onto a word beginning with "p-", such as "nonproliferation" or "unpleasant". But when we pronounce such words, we have to take one of two tacks: either we deliberately enunciate the syllables so that the two consonants are clearly apparent, or we allow them to flow together--as is usual in fluent speech, since "flowing" is the meaning of "fluent". And when we do this second thing, the "-n-" changes: it turns into an "-m-", because "-m-" is so much closer than "-n-" to "-p-" on the palate. Try it: say "rainproof" or "davenport" slowly and carefully, and then try speeding it up more and more, and you'll see that the "-n-" invariably mutates into "-m-". Sometimes it happens so much in speech that it's codified in spelling: "symposium" started out as Greek "sunposion", which turned into "sumposion" as that "-np-" got blurred into the much more fluid "-mp-". (Clearly "-np-" didn't fit the Greek speaker's mouth all that well, either.)
And that is a sandhi: the transformation of one sound into another due to the effect of adjacent letters.
It happens all the time in ordinary speech. "Don't we?" sounds clear and accurate at any speed, but "Don't you?" is rapidly transformed into "Doan chew?" (or even "Doan cha?" if we're speaking really quickly). We can, of course, pronounce everything precisely if we want to--it's no great challenge to pronounce "nonpareil" with all the consonants in place--but in everyday speech, the rules are relaxed a little, our tongues take the easy way out, the morphemes slip and blur, and before we know it we've got new sounds, and maybe whole new words.