Problem with speakers on Zoom (they work fine for general Windows)

I have been using external speakers and webcam for Zoom on my Windows 10 desktop with no problems, until yesterday when in the middle of a Zoom session all of a sudden I stopped getting sound through my speakers. When I try the "Test Speaker" function I can hear a *VERY* faint sound (almost inaudible) if the speakers are turned to maximum volume, but that is all - they are not usable in the current state. For other Windows 10 desktop applications (speaker tests, notifications, listening to media over web browser, etc.) the speakers are working as normal... except for Microsoft Teams where I am experiencing the same problem. I have tried rebooting my computer, uninstalling & reinstalling Zoom - no luck. As far as I can see the settings are all correct. I am running out of ideas; what might cause speakers to suddenly stop picking up Zoom sound? Any ideas gratefully received!

(Extra information: the speakers are connected to my computer via the green headphone jack. I do not have another set of speakers to try, but I have headphones with a USB connection and I can hear sound from Zoom through these.)

Thanks a lot!