90% of the time it's because their speaker device is set wrong in Teams because they plug into a meeting device or something or a dock that has the speakers set wrong for defaults. Check Teams devices and make sure it's the proper output device. I've seen these devices muted in the back ground so make sure to open the sound control panel and make sure it's turned up, and I always make sure to right click the device and set as default communication device for output as this seems to have effect as well sometimes with Teams so next time the audio don't switch to another device like hdmi out or something automatically.
