I know that the group of vowel pairs which form monosyllabic sounds are called dipthongs ( oo, ow, ai, etc.), but what is the term for the group of consonant pairs which form monosyllabic sounds? ( ph, th, sh, ng, etc.)

1) For the examples that you give, I would use the term digraph:
"A digraph, bigraph or digram is a pair of characters used to write one phoneme (distinct sound) or a sequence of phonemes that does not correspond to the normal values of the two characters combined. The sound is often, but not necessarily, one which cannot be expressed using a single character in the orthography used by the language. Usually, the term "digraph" is reserved for graphemes whose pronunciation is always or nearly always the same.

When digraphs do not represent a special sound, they may be relics from an earlier period of the language when they did have a different pronunciation, or represent a distinction which is made only in certain dialects, like wh in English. They may also be used for purely etymological reasons, like rh in English.

In some languages, digraphs are considered individual letters, meaning that they have their own place in the alphabet, in the standard orthography, and cannot be separated into their constituent graphemes; e.g.: when sorting, abbreviating or hyphenating. In others, like English, this is not the case."
Source and further information:
http://en.wikipedia.org/wiki/Digraph_%28orthography%29

If there are three of them, it is called a trigraph:
http://en.wikipedia.org/wiki/Trigraph_%28orthography%29


2) However, the term digraph means something slightly different:
- a digraph can also be composed of two vowels or a vowel and a consonant (for instance a diphtong)
- a digraph represents one phoneme (distinct sound) or a sequence of phonemes, it must not be a syllable, because a syllable needs a nucleus, which is usually a vowel:
http://en.wikipedia.org/wiki/Syllable
http://en.wikipedia.org/wiki/Syllable_nucleus


3) Further information about ph, th, sh, ng:
http://en.wikipedia.org/wiki/Ph_%28digraph%29
http://en.wikipedia.org/wiki/Th_%28digraph%29
http://en.wikipedia.org/wiki/Sh_%28digraph%29
http://en.wikipedia.org/wiki/Ng_%28digraph%29

4) Further information:
http://en.wikipedia.org/wiki/English_orthography
http://en.wikipedia.org/wiki/Pronunciation_respelling_for_English
http://en.wikipedia.org/wiki/Consonant
http://en.wikipedia.org/wiki/List_of_consonants

Phonemes

MobiusDick