You can find a million different circuits online with a simple google search. The problem is you haven't given any useful info about what it is you need. You want to generate a sound outside? Cool, I can point you to a circuit that you can't hear from five feet away, but it meets your criteria because it was used "outside" and produced a garbled mess that still counts as a "sound".

If you want help, you need to provide an idea of how much power needs to be generated, how many speakers you are driving, how clear you need the sound to be (and this distinction will be quite different depending on if you are generating a tone, music, voice, etc), how FAR away you want to hear the sound, and it wouldn't hurt to show what you've already tried and how that varied from what you were expecting. If you plan to leave this outside long-term, then you should describe what your plan is for weather-proofing as that will have an effect out the output also. And since I don't know jack about analog circuits, there's probably more information required to build what you need.

Sorry, even if you want nothing more than a siren sound there's nothing easy about your question.

Like single frequencies/tones (beeps, boops)? Or more complex sounds (dog barking, voice)? A little more specifics about your project would help us give better info.

Before you try a complex diy solution you already have tried a easy solution, like "Animation away" from "Royal Gardeneer" or similar gadgets which create a ultrasound siren?!? And if this 20,- EUR/USD Gadget is not working for these dogs, you could open it and use 99% of the device to add an mp3-circuit-board.

Power supply, speaker, case, and however you want to trigger, generate, and terminate the sound. For simple sound, and oscillator circuit and specific logic. More complex they use to sell sound chips. These days you could use a raspberry pi or even an old cell phone and program it. There is probably even off the shelf stuff you could link with Wi-Fi, z-wave, or zigbee these days.