To add audio to an instant messaging (IM) conversation:
At the bottom of the conversation window, click the Call icon, and then click Call.
Note: If you’re running Skype for Business (Lync) on Windows 8.1, then certain audio or video devices may not work because the device drivers don’t install properly. This is a known issue on Windows 8.1. A possible workaround is to update the device drivers to the latest version.