Adjusting alert preferences for messages, mentions, and activity within Microsoft Teams
The timer notification is configured from the meeting timer itself, rather than from the general Teams notification settings.
During the Teams meeting:
Open the Timer app/control in the meeting.
Start or create the timer.
Open the timer's settings/options menu.
Look for the Sound/notification when the timer ends option and enable it.
Make sure the timer is actually running, then test it with a short duration.
The important distinction is that the Teams Settings → Notifications page controls Teams notifications generally. It does not necessarily expose a separate setting for every in-meeting app or meeting feature.
If you don't see a sound option in the timer interface at all, make sure you're using the current Teams desktop client and that the Timer feature is available in your organization's Teams environment. Features in Teams can vary depending on the client version and tenant configuration.
I would also test it with a 10- or 30-second timer while in a meeting. That will tell you very quickly whether the issue is the timer configuration or the system/Teams audio notification itself.