Microphone keeps switching

Hello

I've never had this problem before but my computer seems to have 2 or 3 audio options that I don't fully understand.

1. Nvidia

2. Realtek

I think that one is for the inbuilt speaker in the screen while another one is when I plug in a mic and speaker probably.

The third option while in Zoom is system.

Anyway, in my Zoom session a couple of minutes ago, a screen kept appearing that the audio output had been changed and the sound kept switching from the speaker in the screen to the headphones every few seconds

I tried forcing Zoom to stay with one system  by making a choice in audio options, but this got overridden and the problem started again

After a few minutes, only the speakers in the screen worked and the headphones were dead which was hard as I had to take the headphones off and then could not easily use the microphone attached to the headphones

But why did I get this problem today? I have a really important Zoom meeting on 14Oct and I need to fix it before then

Thanks for any help in advance