Make it louder.

Mount it higher.

Welcome to the forum.

In this case, I think you are asking the impossible. The volume of a single siren capable of 80dB at 5km (Loud enough to be heard over other potential devices like personal headphones and inside a motor vehicle), I suspect would be lethal for a significant distance from the source and cause permanent hearing damage for a large proportion of the area involved.

I would suggest that a system using multiple interconnected sirens would be appropriate and probably more cost effective.

If you want everyone for 5km to hear, then where is the safe evacuation zone? Could a person on foot outrun the hazard to get to safety?

What are you handling that needs a 5km evacuation zone?

<...everyone around 5km...>

What about those that are deaf?

What you need are large, loud disaster warning sirens.

For example

Be aware that for an area this large you will need multiple sirens and they will need to be mounted fairly high. The exact size, number and placement will depend a lot on the terrain, and obviously the sound is directional (and is reflected and deflected by objects) so that has to be taken into account.

Basics on selection advice

Your best bet is a detailed look on the internet for information as there should be enough available for you to put together a basic design. There may also be helpful design software (similar to lighting design software but for sound), but I don't know of any specifically.

here an interesting link Victory Sirens.