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!

Systemd timer to poll upower when running on battery power, when battery is at 20%, use either system beep or set system volume and play a sound?

I seem to get pop-up notifications for free in GNOME/Fedora by setting these levels in /etc/UPower/UPower.conf:

UsePercentageForPolicy=true
PercentageLow=50
PercentageCritical=20
PercentageAction=10

I think you can also configure the system to take action when it reaches the lowest level with e.g.

# The action to take when "TimeAction" or "PercentageAction" above has been
# reached for the batteries (UPS or laptop batteries) supplying the computer
CriticalPowerAction=PowerOff

However I don't know how to get these GNOME "Power" notifications to play an audible sound (without turning on notification sounds for ALL notifications). The best I could find is this: David Bazile / gaudible · GitLab

There's talk of better control of sound notifications in GNOME 47+, but looks like nothing much has landed yet: Notifications in 46 and beyond – GNOME Shell & Mutter

The arch wiki has a udev rule that can automatically do something if the battery crosses a certain threshold: https://wiki.archlinux.org/title/Laptop#Hibernate_on_low_battery_level

No polling which is great. I always try to do stuff on an event driven basis where possible for efficiency reasons. Gotta test this out though, since your battery might not send events for every percent change.

I would instead look into the firmware settings. Dell has good Linux support.

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.