hiya,
"xi" in mandarin chinese has a pronunciation similar to the sound of the word "sea".
"ao" in mandarin chinese has a pronunciation similar to the sound of that common comments off our mouth "oww" (oww, that hurts).
but know that all chinese words, when using fragmented english words to represent them, has four different ways to pronounce them.
it's like doing "lalalala" in base minor or suprano (an exagerrated e.g.).
"xi" is like a base word, by varying how it sounds, different words and ideas are thus projected.
"xiao" will sound like "ciao" or "sea-ow" (somewhat low/bass like tone).
too high a tone used for pronouncing "xiao" will result in the meaning of "laugh", "flute", "school" or etc rather than "small" (i think small is the intended meaning right?)
hope it helps.