Learn Icelandic: Þinn hljómur er ekki frumsaminn.

This relates to focus. Placing the possessive pronoun before the noun (Þinn) adds strong emphasis, contrasting your sound with someone else's sound. Notice that when you prepose the possessive like this, the noun (hljómur) must be in its indefinite form, completely dropping the -inn article.

Would the meaning change if I wrote Hljómurinn þinn er ekki frumsaminn?

The core translation remains the same, but you lose the contrastive emphasis on the word your. Hljómurinn þinn is the neutral, everyday way to express possession in Icelandic. The preposed version in this sentence sounds slightly more pointed, formal, or argumentative.

How is the word frumsaminn constructed?

It is a compound word made from the prefix frum-, which means primary, first, or original, and saminn, which is the past participle of the verb semja, meaning to compose. So it literally translates to originally composed.

Why is it frumsaminn and not frumsamið here?

Adjectives and past participles must agree in gender, number, and case with the noun they describe. Because hljómur is a masculine singular noun in the nominative case, the predicate adjective describing it must also take the masculine singular nominative form, which is frumsaminn.