Greetings!
This is Tin, sorry to hear that you can't adjust the volume of the sound from your device while attending webinar. Kindly check and follow the steps below:
May I know what application are you using to attend the webinar? Have you tried to check the volume from there?
Please try the FN key from your keyboard, usually it is in F1-F6, please look for the speaker icon.
If the issue persists, Run the Audio troubleshooter
Click on the Start menu and type Troubleshoot and then press enter
Find the Audio option and then run it
Check if the problem persists.
Please let me know how it goes.
Thanks,
Tin