FreeCorder has a switch in its Settings that enables you to record Skype calls - this essentially opens both the Mic and another port for recording. You may use this idea to develop your own software.

p.s. Skype uses a proprietary protocol, so it might be hard to record from it.

p.p.s. You may also record Skype calls using Replay Telecorder . It allows to record both audio and video calls from Skype.