My Prusa i3 does not appear to have any sound device incorporated. I've read "Writing G-Code: Audible Alert at end of print" and it seems everything needed to implement audible alerts is built into the G-code syntax and firmware but my rig appears to be lacking the necessary noise maker.
I just looked in the Marlin FW
pins_RAMPS.h where the pin assignments are located:
#define BEEPER_PIN 37
Please check for yourself. I assume you run Marlin, but my version is old and may not be correct.
As you mentioned there is no LCD on your printer so most likely this pin is free, just need to find it.
Also, just a warning, use a proper buzzer or beeper module with the onboard transistor. Do not connect a buzzer directly to the signal pin as the current draw can kill your controller.
Possible is this an idea found at the octoprint web site?