Same /i/ sound: only the spelling changes. After the linking u and after qu, write y (tuy, quý). In some words i and y are free variants: kỳ or kì, lý or lí. Not to be confused with ai / ay, where the length of the a changes.
tuy · quý · kỳ · kì
ai or ay? ao or au?
In ay and au the a is short (an ă written a). In ai and ao it is long. Listen to the length: hai / tay, sao / sau.
hai · tay · sao · sau
Quick answers
How do you pronounce xin chào?
Block by block: i → n → in → x → xin · a → u → ao → ch → chao → huyền → chào. Tone: xin: ngang (no mark · level) · chào: huyền (grave accent · low falling). Read each block aloud, then compare with the recording, in a Southern or a Northern voice.
What tone is xin chào?
xin: ngang (no mark · level) · chào: huyền (grave accent · low falling)
Is xin chào said differently in the South and the North?
South · Sài Gòn: x = s, in = inh = .i.ŋ, ch = tr. North · Hà Nội: x = s, ch = tr.