How to ask Chinese tourists keep their voice down?
What is the best sentence to let them know that they are making very loud noise?


Before I give out example sentences for asking people to lower their voice, I have to warn you-- Telling strangers that their behavior is unacceptable in public might lead to verbal confrontation or worse. 


Some people might think talking loud is their personal right, telling them otherwise is an intrusion.  No words of yours can change their behavior.
Some people might not notice they are being loud. They can be reasoned with, but you have to be extremely polite. You are pointing out something they do wrong after all. No one like to be criticized.


Now there are some example sentences you could use.


Start with "對不起" /duìbùqǐ/ (excuse me/ sorry).
Then beg them "能請你們" /Néng qǐng nǐmen/(would you please).
Tell them what you want, "降低聲量嗎?"/Jiàngdī shēngliàng ma/ (lower your voice?).
(optional) Make them think you are with them "外国人爱挑剔别人大声说话" /Wàiguó rén ài Tiāotì biérén dàshēng shuōhuà/ (Foreigners love to pick on people who use loud voice).
Thank them for cooperating "谢谢合作" /Xièxiè hézuò/. After being thanked, most reasonable people would not be able to get too angry with you.