How can I get sound alerts for low battery on iPhone?

You get a warning display at 20%. When you get that you should charge it NOW! You get a second warning at 10%. Of course, you have to look at your phone to see these, but if you aren’t actively using the phone the battery drain is very low, and it won’t shut down. Unless you ignore both warnings.

Go to Settings/Battery/Battery Health and see what it says; if it says your battery is degraded, or if max capacity is below 80%, replace the battery→iPhone Battery Replacement - Official Apple Support

There are third party apps the will play an alert sound and/or send notifications.