How can I say "your voice is lovely"? Or something similar?
Well it's for a girl , have you heard Japanese girls they usually fake their own voices! I hate that !! She speaks English & Japanese ,but I don't want to ask her directly you understand? I want it to sound a little polite 😁Similar to what? You haven't provided any context, without which we can't suggest other options. What is the situation? Who is talking to whom (what is their relationship)? Are you referring to a speaking voice or a singing voice?
Both of those sound good, but one or the other might be preferable depending on the exact context.
You can also switch around the word order:
And if the situation called for a more polite form, you could use お声.
Japanese kawaii (unlike English/etc. kawaii)
Excuse me, but:I don't understand what you mean by that, since "kawaii" does not exist in English or any other non-Japanese language that I am aware of.
If you are simply trying to say that many 外国人 do not understand how to use the word かわいい（可愛い）properly, then there are much better ways to express that idea.
As your examples demonstrate, context is crucial here, but the OP still has not provided a detailed description of the situation and a full sentence or dialog, so it's impossible to give a good suggestion.