Hi there!
I see that it's been a while since you reached out to Community with this issue. Have you found a solution yet? If so, feel free to share here so others might benefit from this knowledge.
It could be that in the type of webinar you experience this issue, the webinar overrides the phone's volume buttons. Zoom client sound settings can be tested and adjusted to your preference. See:
Test Your Audio Settings (Zoom video)
Testing audio before Zoom Meetings
If my reply helped guide you in the right direction, don't forget to click the Accept as Solution button! ⤵️