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
hỏi or ngã?
Two different marks in writing (hook / tilde). In the South they sound the same: only knowing the word tells you which.
ngủ · phở · sữa · mũi
Quick answers
How do you pronounce xin lỗi?
Block by block: i → n → in → x → xin · ô → i → ôi → l → lôi → ngã → lỗi. Tone: xin: ngang (no mark · level) · lỗi: ngã (tilde · broken rising). Read each block aloud, then compare with the recording, in a Southern or a Northern voice.
What tone is xin lỗi?
xin: ngang (no mark · level) · lỗi: ngã (tilde · broken rising)
Is xin lỗi said differently in the South and the North?
South · Sài Gòn: x = s, in = inh = .i.ŋ, ngã = hỏi. North · Hà Nội: x = s.