Method 1: You can use some virtual audio device, like this one. After installing it, you will see a new recording device in Control Panel (mmsys.cpl) and Lync will allow you to join the audio call.
Method 2: According to this article, you can trick Lync/S4B without installing any software but you still need admin rights to edit registry:
- Insert your headphones into microphone plug for 10-15 seconds to initialize it (the microphone) in drivers.
- Open regedit.exe and proceed to HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\MMDevices\Audio\Capture.
- Find the subkey with DeviceState = 0x00000008 and change it into 0x00000001.
- Check for a new recording device using "control mmsys.cpl,,1", start Lync and join the audio call.
Method 3: If you don't have admin rights, just put some old headphones into the mic plug.