First, make sure your microphone is not muted in the Teams call. Look at the bottom center of the call screen
Second, make sure your speaker is not muted. Click the speaker in the taskbar and make sure there isn’t an X to the right of the speaker.
If both of these items are ok and the problem still persists, please proceed.
Open Teams if it’s not already open.
Click the three dots in the upper right and select Settings
Click Devices and look at Speaker and Microphone. The choices here will vary depending on what you have set up on your computer. Some users have an external headset and other only use the internal speakers and microphone of their computer. If you are using a headset, the headset should be selected for both items. If you are using the internal speakers and microphone of your computer or Surface those should be chosen. Other items that can have a microphone or speaker could be a monitor or an external monitor. Usually those are not good choices except in rare instances where other options are not available. Change these items around depending on what the problem is. If they can hear you but you can’t hear them, the the Speaker selection may be wrong. Look at that. If you can hear them but they can’t hear you look at the Microphone.
You can test your settings by clicking the Make a test call button.