Do you change your voice when you're surprised, unsure etc. in Mandarin?
Well in China,we use the same way to pronounce words in sentences.We may speak louder with angry tone when we are angry or quarrlling with someone.

For if changing tone may at the same time change the meaning,I want to say that it doesn't matter when you try to understand the meaning of the whole sentence.If you pick the emphasized word out of the sentence,it may sound very strange or just mean another meaning.Well in most cases,it's also recognizable because there are only four tones in chinese and the possible combination is limited,and to express common meaning,the quantity is even decreased to an easily counted number.So most time,we can get what the speaker said.

Well there is another question,that is the accent!It's not in the range of mandarin while I still want to say some.In China there are a large number of different areas.So sometimes even the pronunciation may be changed totally.It's common in China that people in one area can't understand what people in another area radically for historic and territorial reasons.Very interesting for the fact but bothered for communication,isn't it?