Xx is a somewhat special letter in written English because it does not represent its own unique phoneme. In fact, it often serves to represent more than one phoneme in a word. For this reason, instructors may devote time in an introductory cued English class to covering how words with Xx are cued.
The two most common pronunciations for the letter x are combinations of /kkkkkkkkk/ and /sssssssss/ as in Texas, mix, flax, and axis or /ggggggggg/ and /zzzzzzzzz/.
Notice that the addition of /sssssssss/ or /zzzzzzzzz/ follows the same voicing rule that occurs with addition of plural -s.