Whoever disabled it didn’t do their due diligence, because if the car were to hit someone blind without the pedestrian warning sound they would liable for consequences. They’re required for any hybrid or electric car traveling at low speeds (below 20 mph). To answer your question the previous owner probably just unplugged the speaker so you’d just have to find out where it is on the Avalon and plug the connector back in.

I have this exact same car and there’s a back-up noise when in reverse. There’s also a noise when it’s moving forward under 21 mph for ADA-related pedestrian safety.

With no source to back me up, I thought all required it as you wouldn’t be able to hear it so well without the backup noise. My wife’s '22 RAV4 Hybrid does.

Maybe someone disabled it?

they are. if bought used, either the last owner disabled it or the speaker is broken.

Is it operating on battery when your backing up? The noise is only required when the engine isn’t running.

One thing to note- if the engine is running, the sound doesn’t come on. I know this time of year a lot of people have the heat going which means the engine is more likely to run and no backup noise.