Ok, I managed it by myself after a bit of tinkering. This is the bash script:

#!/bin/bash
while true
    do 
        battery_level=`cat /sys/class/power_supply/BAT0/capacity`
        battery_status=`cat /sys/class/power_supply/BAT0/status`
        if [ $battery_status = "Discharging" ] && [ $battery_level -lt 21 ];
        then
            /usr/bin/aplay  /home/eugenia/Music/alarm.wav
        fi
       sleep 120
    done

Obviously change the path the .wav audio file to suit yours (I downloaded mine from the internet). Then, save the file (in my case, I named it battery.sh), make the script file executable ( chmod +x battery.sh via the terminal, or via the file manager).

Then add it to the Startup Applications settings panel on your distro (usually gnome and cinnamon have one). The alarm will sound if the battery reaches below 21%.

You could look into using scripts with tools like acpi or upower. A simple shell script checking battery levels every few minutes could work: if it’s below 20%, play a sound. Schedule it with a cron job or a systemd service for consistency. I'm no script guru, but there's lots of good examples online!

I would first attempt to fix the problem by flashing the bios with the newest version. What you are describing sounds a lot like a common issue with how deep sleep works. Its on the manufacturer to fix it which Dell is typically pretty good about.

If that doesnt work for some reason you can get an applet that will shut down your pc when the battery gets too low for you. Theres a few that work just right click the panel and search for battery applets.