I think she's refering to a transliteration symbol- the traditional symbol for that sound is an h with a curved line underneath it, but since that is not available on the keyboard a lowercase x is used in computer transliteration. The actual 'glyph is a placenta- a circle with several lines through it. It makes a sound like the /ch/ in the Scottish word, "loch."

There is another "x" sound which is similar- the traditional transliteration is an h with a straight line underneath it, and the computer transliteration is an uppercase X. The glyph is a view of a mammal's belly from the underside- you see the belly, rows of teats on either side, and the tail streaming out behind. It sounds like the /ch/ in the German word, "Ich."

There's a really wonderful guide to the pronunciation of basic Middle Egyptian phonemes here, which goes over all the uniliteral signs and has sound files to help you hear the pronunciation. cool