Resolved No sound from monitor speakers?

I apologize if I'm posting this in the wrong place, but wonder if the Win7 OS may be the cause??
My elderly mother has this operating system on her desktop computer (yes, I know it's old & no longer supported) but she refuses to change to anything else as she fears she won't understand any of it.
Anyway, she has HP monitor with built in speakers (I'm not sure of the model) connected to it. I was visiting one day and wanted to show her a funny video clip and discovered she wasn't getting any sound!
Have tried reading on the internet to find out why, but that just basically confused me. I did bring a headset there to test & the sound definitely works through those!
She never noticed/realized that her speakers weren't working as she never really listens to anything audio, just reads.
Any idea what the problem could be? Her sound is set to "high definition audio" if that matters?

Thanks!