Learn Russian: Звук всегда очень звонкий там, где хорошая акустика. - The sound is always very resonant where the acoustics are good.

In English, the word where can connect these two clauses directly. In Russian, spatial relative clauses usually require a matching demonstrative word in the main clause to anchor them. Here, там (there) points to the location, and где (where) introduces the description of that location. Dropping там would sound grammatically incomplete in standard Russian.

The English translation says acoustics are, but хорошая акустика is singular. Is this word always singular in Russian?

Yes. When referring to the sound qualities of a room, or the science of sound, акустика is a singular feminine noun in Russian. English often uses the plural acoustics for the same concept, but in Russian you must use singular modifiers like хорошая.

What exactly does звонкий mean? Could I use it to describe a person's voice?

звонкий comes from the word for a ringing bell and means ringing, sonorous, or clear. It describes sounds that are bright and carry well. You can definitely use it for a person's voice (звонкий голос) or a laugh (звонкий смех). As a bonus for language learners, it is also the linguistic term used for voiced consonants.

Why is there no verb in either half of this sentence?

Both clauses describe a present state, so the present tense form of the verb to be is omitted. The first clause is literally sound [is] always very resonant, and the second is where [there is] good acoustics. This zero-verb structure is standard in Russian for present-tense descriptions, but seeing it twice in one sentence can sometimes throw learners off.