On my 8 year-old Dell Studio540 Desktop PC, my Setup Guide has this to say about the Blue Jack:
5) Line-in connector — Connects to a microphone for voice or audio input into a sound or telephony program. On computers with a sound card, use the connector on the card.
It would also be helpful to have the Make/Model of your computer so we can look up what the Blue Jack does on
YOUR model computer, as neemo suggests.
Hope that's helpful.
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