You need to distinguish between sounds and letters. For English especially, they are very very different. Writing is a technology that has been developed to represent language, but writing systems vary hugely in how they go about representing a language, and how successful they are at doing so.
A silent letter has nothing to do with phonology: it is a part of spelling. Often it is a way of signalling the particular sounds to the reader (eg "silent E" in English), but sometimes it has no effect on reading and is simply a historical accident (eg the K in "knife": it is not sounded, and does not distinguish the word from any other word).
A diphthong is a vowel sound which changes during the course of its utterance. It happens that the word "break" is pronounced with a diphthong /ɛɪ/ in many varieties of English (not all) but that has no relation to how it is spelt: "make" has the same sound. A letter pair like "ea" is a digraph. (It is true that some people refer to it as a diphthong; but in a discussion about phonology it is important to distinguish between sounds and writing). "ou" in "cough" is a digraph which represents a simple vowel, not a diphthing (in all varieties of English, as far as I know). Edit: and "ea" itself represents a simple vowel sound in "bread".
/k/, /m/ and /b/ (the sounds) are called "consonants". /m/ and /b/ are nearly always written with the letters "m" and "b" respectively, and you can call those "consonant letters". /k/ is more complicated: the sound can be written "c" (as in "cat"); "ck" ("back"); "ch" ("Christian"); "qu" ("opaque"); and while the letter "k" is usually pronounced /k/, it is sometimes silent.
I don't think there is a special name for letters that aren't silent in English spelling.