“Sound” when it relates to water comes from Germanic- and Proto Indo European languages. In Denmark, the Øresund (English translation “The Sound”) separates Denmark and Sweden. In Dutch, “zond” used to a be term for a sea-based water inlet into the lands. Many nordic languages still have “sund” in their vocabulary (Danish, Swedish, Icelandic, maybe Norwegian too?).

In Proto Indo European, the “swem” prefix is related to things of the water, or swimming. “Zwemmen” in Dutch still today means “to swim”.

Wiktionary gives the follow definition:

long narrow inlet, or a strait between the mainland and an island; also, a strait connecting two seas, or connecting a sea or lake with the ocean.

It also quotes a text from 1605:

The Sound of Denmarke, where ships pay toll.