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 pronunciation 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. Some words may may be pronounced with either combination. For example, exit may be pronounced with /ks/ or /gz/. Which combination is used may be a function of one's dialect or the part of speech. For some cuers, one combination may be used for the noun and the other for a verb:
The exit is at the end of the hall.
We should exit the building immediately.
Notice that the addition of /sssssssss/ or /zzzzzzzzz/ follows the same voicing rule that occurs with addition of plural -s. So, while a word may be pronounced with /ks/ or /gz/ it is unlikely to mix the combinations. It would not be likely to use */kz/ or */gs/.
ax, axis, box, deluxe, fax, fix, flex, Mexico, mixer, pox, sexy, taxes, toxic, wax