When a UPS/no break is doing the «backup» because the electricity is not present, it starts doing a sound which is kind of anoying, you can turn it off by pressing the Mute button, but some models don’t have this button.
If you want to MUTE the sound forever you have to do the next:
If you don’t have the knowledge or skills in electronic/electricity or don’t have the required tools do not try this, it can be unsafe for you and others and cause damage or death.
First TURN OFF the UPS and disconnect from electricity and remove the battery, usually is removing an small cover and it depend of the model.
remove all the screws from the UPS and remove the cover
Find the buzzer which is an small «speaker» like this:
Find in the other side the pins of the speaker
heating the solder and use a desoldering pump to suck the solder.
Remove the speaker, it will look like this
re-assembly the box
With this the UPS will no longer do a beep/sound