Program request

Does anyone know a program that allows audio to be streamed through the input jack?

Ex: let someone in a skype conversation listen to vent without having to be in it.

Yes.

Get a normal headphone cable Y adapter and a short piece of headphone cable with male ends. Plug the Y adapter into the speaker output, plug your speakers and headphone cable into that. Plug the other end of your headphone cable into the microphone jack. You'll need another splitter if you wanted to talk on Ventrillo at the same time.