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 most common pronunciations for the letter x is a combinations of /kkkkkkkkk/ and /sssssssss/ as in Texas, mix, flax, six. Another common pronunciation is the combination of /ggggggggg/ and /zzzzzzzzz/ as in example and exist.
Notice that the addition of /sssssssss/ or /zzzzzzzzz/ follows the same voicing rule that occurs with addition of plural -s.
ax, axis, box, flex, Mexico, pox, sexy, taxes, toxic, wax