About recording a podcast without VoiceOver output

Hi there. First of all, excuse my poor english. I am from Barcelona, Spain, and I used to read in english, not write. I am trying to record a simple podcast, I mean, only voice and music background. THe question is, I connected my mac book Pro (it plays the music background) to an external sound device, in witch I have connected mi microphone. Then, I plugged the output signal back to my mac to record both music and voice channels both properly equalized and mixed. With this method, obviously, Voiceover is recorded too, for example, when I select another song, or press a keystroke or navigate a menu bar, or when starting a Skype call, in withch I wanna record the calling sesion, but not voiceover saying "contact list... call button... etc". How can I select another song or navigate my mac without this effect? Any suggestion? Or I am doing wrong all the process... Thanks in advance.