Most of the letters are ones that don't look as if they could have their own pronunciation (an umlaut over an e?), and I don't know of any "official" ones. The ones I usually use are:
ű - uh (rather than oo for a regular u)
÷ - as in German (something like an "er" mixed with an "o"... hard to explain)
˝ - the "ni" in "onion"
ŕ - like the "ay" in "day"
Of course, those are entirely my own ideas and aren't official in any way, shape, or form.