When placed after the noun like normal, the adjective carries a fairly objective, descriptive meaning. When placed before the noun, the adjective carries a more subjective, opinionated meaning.
So it´s more like i think (or to my mind) her voice is beautiful. And if we place it after voz it will be something like at one time some competent committee decided that her voice is beautiful and everyone in the world agreed with it)
"b" and "v" often sound alike, depending on the speaker and their nation of origin. Mexican school children spell words wrong because of this ;) Opinions and "rules" vary, but the reality is one must get used to hearing B/V pronounced as a little of both and rely on context, sometimes, to figure it out. Like "f" and "ph" in English. If you say "v" as "v" and "b" as "b" people will understand you, at least, even if you don't pass for a native.
I agree with rspreng, but here are more thoughts on the matter. Although there are exceptions, I've generally observed that if a b or a v is at the beginning of a word (and definitely at the beginning of a sentence), it usually sounds more like b. So, the name Victor often sounds like Bictor. In the middle of a word, though, it sounds like v, such as Lo sabia, which sounds like lo savia. One exception in the middle is if the v follows an n. Then it still sounds like b. For example, enviar, sounds more like enbiar. And the truth is, both the "hard" and "soft" b or v sound sort of in between the way English speakers pronounce them.
There is a fairly full description of the use of B and V at http://www.elabueloeduca.com/aprender/lengua/ortografia/reglasdelab.html., though, of course, it doesn't really help with the sounds.
It's a matter of geographical variations . For instance here in Spain we don't have a different phonem fon b and v it all sounds like the english "b". In Latin america , many countries do make a difference just like the english(b/v). I guess they have kept that from the old colonial times.Hope it helps.
See earlier comments posted above. In general, if the adjective (bonita) comes after the noun, it's objective and non-emotional, and if it's before the noun (una bonita voz), it's opinionated, emotional, etc. So the speaker appreciates the woman's voice, that's opinion, so it sounds better to say una bonita voz.