I've still doubts about how this letter sounds. The online reference grammar
says "x [x] an unvoiced velar fricative".
I can also find:
[x]
The preferred pronunciation of Lojban ``x''. Not normally an English sound,
but used in some pronunciations of ``loch'' and ``Bach''; ``gh'' in Scots
``might'' and ``night''. The German ``Ach-Laut''. To pronounce [x], force
air through your throat without vibrating your vocal chords; there should be
lots of scrape.
I'm still unsure on which sound it might be, but I've guessed it's the same
"H" sound used in Klingon (as in /baH/ or /'oH/). Could someone who knows
both languages (if any) clarify?