[yahnhgmm] Two th

The digraph th is pronounced two different ways in English, but there is no common way of spelling or denoting the two different sounds using only English letters (not resorting to International Phonetic Alphabet) for the purposes of distinguishing them.  This seems strange, especially given how common th is in English.  Are there other common "unspellable" sounds?

The first sound th makes is the voiceless dental fricative.  "Th" as in thin.  (What are some good examples of it within and at the end of a word?  Th seems to become optionally voiced a lot.)  The IPA symbol for this sound is θ "theta" U+03B8 .  When using only normal letters, this sound is typically denoted as "th", e.g., "Spanish Spanish pronounces z as th."

The other sound th makes is the voiced dental fricative.  "Th" as in this, father, and tithe.  The IPA symbol for this sound is ð "eth" U+00F0 .  (Incidentally, how is "eth" pronounced?)  "dh" would be a good way to spell this sound using only normal letters, but it is far from common.

"Either" and "ether" (aether, æther) are two words that sound the same except for their pronunciation of th (assuming you pronounce the first vowel the same; there's a song about that).  Others?

Consider respelling all words using "th as in this" with dh.  "Dhe" would become one of the most common words.  But there are many cases in which a word can have it unvoiced or voiced.  There might be (there probably is) some complicated rule determining whether it is voiced depending on context.  A word ought not be spelled differently even if it is pronounced two different ways.  ("a" and "an" are (arguably) the same word, pronounced and spelled two different ways.  We don't want more like this.)