ある as a pre-noun adjectival is used to mean "one" or "a certain," in common constructions such as ある日.
In this case, ある is clearly bringing some "baggage" with it in the form of:
(Where the の is converted from が). So you can be certain that ある is acting as the verb in a phrase modifying the noun 声.
Having said that, I don't think there's any ...
「人間ができている」 means "(someone) is a mature person".
So you should parse it like:
A man [who is kind and at the same time mature], [like Yoshida-san]
「吉田さんのような」 connects to 「（優しくて、かつ人間もできている）男性」.
優しくて、かつ人間もできている modifies the noun 男性. 優しくて is the て form, ie the continuous form of 優しい, "is kind, and...". かつ means "besides" "...
I would say "yes, the first noun is acting as an adjective"
My hesitation ("would") comes from realising that we may be taking about grammatical definitions here (so I don't know how appropriate the "like an adjective" is), but if someone posted the following question on a Japanese forum that focuses on people "researching" the English language:
In Japanese, the use of particles for time references is generally predicted by whether the time reference is fixed or relative. For fixed time references, the particle に is used to mark the time word. For relative time references, the particle is usually omitted.
Examples of fixed time references are named days, times, dates, years, etc.
Examples of ...