So, у тебя есть is more for something one can own or have, an object or person, (you have a ball, you have a big brother) but у тебя without the есть is more for a personal trait or something that physically would belong to you (I have a soft voice, you have smooth skin?
I am not sure I understand your question. Maybe this will help:
У Ильи седая борода = Ilya has a grey beard.
У Зины красивые руки = Zina has beautiful arms/hands.
У Миши кудрявые волосы = Misha has curly hair.
У Петра большой живот = Petr (Peter) has a big belly.
У Софьи хорошее чувство юмора = Sophia (or Sofia) has a good sense of humor.
Placing the modified noun in any of the above sentences at the beginning of the sentence will change the sentence as follows: Борода у Ильи седая = Ilya’s beard is grey. etc.
It depends. You can say, for instance, «У меня руки в масле» (My hands are oily) or «У меня руки чешутся» (literally: “My hands are itchy”, or, figuratively: “I’m itching/desperate/tempted [to do something]”/“I got half a mind [to do something]), or «У меня большие руки» (I have big hands) or «У меня руки закоченели» (My hands are nearly frozen), but you cannot simply say, *«У меня руки» — this sentence is incomplete. It is OK, however, to say, «У меня есть руки», which basically means something like “Why, I can do it — after all I’ve got two hands”.
I agree with maiqilaiXVX. As a native English speaker, I don't perceive much, if any, difference between the meaning of "quiet voice" and "soft voice," but "soft voice" is much more commonly used. From your comment, it sounds like there may be more difference between "тихий годос" and "мягкий голос" and we can't use literal translations.
No way. Only a person can be described as молчаливый, because the word comes from the verb молчать (to keep one's mouth shut). Молчаливый человек - это человек, который всё время молчит. "Silent voice" doesn't make sense to me. Does this mean something like "красноречивое молчание" (eloquent silence)?
You sure can. I think “a soft voice” sounds much better than “a quiet voice” when talking about someone’s permanent characteristic as in the given sentence. For a particular occasion тихий голос means “a low voice”. “In a low voice” can be translated into Russian either as «тихим голосом» or as «вполголоса».
Adjectives in which the stress falls on the ending (in the case of a two-syllable ending, on the first syllable of the ending) end in the Nominative in -ой, -ое, and -ая, in masculine, neuter and feminine singular forms, respectively. Such adjectives are fewer than the ones with unstressed endings; however, there are a few dozens of them. Here are some examples: большой, голубой, тугой, плохой, прямой, хромой, слепой, глухой, родной, грибной, лесной, ручной, речной, глазной, ушной, печной, свечной, мучной. In cases other than nominative and inanimate accusative, the distinction between masculine and neuter disappears.
«У тебя громкий голос» means “You have a loud voice” where the key word is “loud”. So basically it means “You are vociferous”. Nobody ever says, «У тебя (У него /У неё) есть громкий голос», because that would imply that the person you are talking about does have the means to solve a particular problem and that means is the person’s loud voice. I can’t think of a problem which could be solved by shouting, so the sentence with есть is very unlikely. So it is OK to use есть in the absence of an adjective, but when the noun is modified, it is the adjective that matters and the word есть should be dropped out.