There are words where one letter refers to two different sounds, e.g. eighth, where T exists itself and also makes a /θ/ sound, or threshold, which is sometimes pronounced with a /h/ sound, especially in AmE. H in this word also forms a /ʃ/ sound.
So my question is: what are such words or this phenomenon called?
If the individual sounds are phonemes, as represented in the question, then the individual letters used to represent them are referred to in linguistics as graphemes. I have heard combinations of graphemes which represent a single phoneme, which is the phenomenon you're asking about, as multigraphs - I don't know if that is common current use.
As you are no doubt aware, there is no one-to-one relationship in English between phonemes and either graphemes or multigraphs. 'Eighth' is a particularly interesting example, where the first four graphemes represent - depending on accent - either a single vowel sound or a diphthong, and the remaining two appear to simultaneously represent two different sounds, as you can't assign the /t/ sound to the <t> grapheme without breaking up the <th> multigraph, so while 'multigraph' covers one part of what you're asking about, it doesn't cover the specific situation you referred to.
One source uses the phrase "combined sound" to refer to the "ts" sound the letter "c" makes in words like "currency" and "council." https://www.teachphonics.co.uk/c-group-alphabet.html
A digraph is the opposite phenomenon, where two letters make one sound. We're looking for the name when one letter makes two sounds.
The letters R and L have 2 pronunciations at the beginning and end of the words ‘rider’ and ‘label’ for example. (Note tongue placement when you say the words). The first sound in each example is called ‘clear’ and the second ‘dark’.
