Skype for Business Web App automatically detects and selects the most compatible audio device for your meeting. If you want to change your audio device, go to Call controls > Devices and select the device you want.
You can also save your audio device preference for future meetings:
Go to More options > Meeting options.
On the General tab, make sure the Remember me on this computer box is checked.
On the Audio tab, click or tap the current device to see other available devices.
Choose the one you want and select OK.