Learn Turkish: Bu müzik kim tarafından bestelendi? - By whom was this music composed?

Tarafından is the standard way to say "by [someone]" in passive sentences when referring to a specific person. The suffix -ce (or -ca) is usually used for groups, institutions, or collective authorities (like hükümetçe - by the government). Since kim (who) asks for a specific person, kim tarafından is the natural choice.

How is the passive voice built in the verb bestelendi?

The root noun is beste (musical composition). We add -le to make it a verb: bestelemek (to compose). To make it passive, we add the passive suffix -n (which is used after vowels), making it bestelenmek (to be composed). Finally, we add the past tense suffix -di, giving us bestelendi (was composed).

Could I change the word order to "Kim tarafından bu müzik bestelendi?"?

You could, but it sounds less natural. In Turkish, the most important piece of new information (the focus of the sentence) usually goes immediately before the verb. Because the core of your question is who did it, kim tarafından belongs in that pre-verb spotlight.

What is the literal breakdown of the word tarafından?