How to share an audio device between many messenger applications?

[Q] How to share an audio device between many messenger applications,
like msn/skype/gtalk?

[Q] 如何讓msn, skype 使用同一組麥克風/喇叭 共存 ?

[thinking] must sacrifice sth, let the audio device be tied to the most recently active messenger application, that is, the backgnd one cannot have sound alert out.