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.
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.