What literary devices is used in these lines from "Music, when soft Voices Die (To--) " by Percy Shelley?

I believe the answer is: Metaphor

Metaphor is a literary technique that is used to describes a person or object by referring to other things that have similar characteristics.

In the line above, the similar characteristics rely on the part when music ends with something that dies.

If the options are metaphor, alliteration, allusion, and apostrophe, I believe the answer is metaphor, because the others don't fit.

Alliteration is the repetition of a consonant.

Allusion is reference to something or somebody outside of that literary work.

And apostrophe is the author speaking directly to somebody outside the poem, usually gods or a muse.