Learn Russian: Звук был где-то недалеко.

The word где means "where". Adding -то makes it "somewhere" (где-то). We use -то when a place actually exists or an event already happened, but we just don't know exactly where. Because the sound was heard in the past, it definitely came from a real place, so we use где-то. If you were talking about a hypothetical future or asking a question like "Can we go somewhere?", you would use где-нибудь instead.

Is the hyphen in где-то mandatory?

Yes! The particle -то is always attached to its question word with a hyphen. This is a strict spelling rule in Russian for words like кто-то (someone), что-то (something), and где-то (somewhere).

Why is был masculine here? Since a sound is an "it", shouldn't it be neuter (было)?

In Russian, past tense verbs must match the gender of the subject noun. The word for sound, звук, ends in a consonant, which makes it a masculine noun. Therefore, the verb must be the masculine form был, even though it translates to "it was" in English.

Can I use близко instead of недалеко to mean "nearby"?