"Are those people young?"
Translation:Jsou ti lidé mladí?
I'm native AmE, so you may get a better answer from a Czech native. But I would say that the sound in ty is like the English word "tip," while in ti, it is a little softer, as if there were a slight "ye" sound between "t" and "i." So the pronunciation of ti would be somewhat more like "tyip." The difference between ty and ti is similar to the difference between the pronunciation of "tune" in AmE ("toon") and BrE ("tyoon"), though perhaps more subtle.
The voiced version, Ď, you can hear in the word mladí here. And also at https://forvo.com/word/mlad%C3%AD/#cs
Please see my answer at https://forum.duolingo.com/comment/31997939 and then accept that an agreeing modifier (and adjective that is placed as a modifier to a noun and follows its grammatical case) must be before the noun. It is the same way as in English. "young people" and not *"people young", "mladí lidé" and not *"lidé mladí".