Does one use an or a before a word beginning with x?

In a dictionary, the pronunciation guide for the word "X-ray" is: eks ˌrā

Notice what the first letter of the pronunciation is e, which is a vowel, thus a vowel sound is the first sound produced when anyone pronounces the word X-ray correctly.

The letter x can begin with either a vowel sound or a consonant sound depending on the word. In certain words that begin with x, the initial sound of the x is actually a z sound:

xylophone is: zīləˌfōn Therefore, a xylophone would be correct.

Use the a if the x is pronounced as a consonant; use an if the x is pronounced as a vowel.