The expression predates steam and sailing ships, by and large
didn't have 'speaking tubes' because it wasn't necessary. Who
were you going to talk to in the cargo hold anyway?

Interesting hypotheses.

Absolute guano, but other than that, interesting.

If that doesn't convince you consider this: the songs (various
versions) refer to the Black Ball Line and the Flying Fish line.

Both pre-date steam, and the associated speaking-tubes used
to communicate with the guys in the engine room.

The song dates from the heavy-hauling required on packet
ships under sail.

So does the expression.