JF Ptak Science Books Post 2564
I posted this thinking that I had never heard the sound of a gas warning siren--or any other noise-making instrument--signifying the onset of a poison gas attack during WWI. So I went poking around the web, looking for an audio recording of one. I did find contemporary audio of an antique instrument--this was a dreadful sound, sounding somewhat like a high-pitched pulse-jet engine. It is an awful sound (linked below) though I am sure that it did its job very effectively.
I haven't yet found a contemporary recording, though I've got a notion that if no audio recording was made for training films and such that they may exist in movies.
Sirens of course was one method of alerting troops to a gas attack--there were also bells of all shapes and sizes, and kettles, wooden clappers, rattles, empty shell cases, and other such things.
Here's a compelling image of conflicting emotional input, this showing a kettle with a clanger attached by a wire: