logitech x 540 speakers

Look at the option audio jacks on that model to see which is for mic, line in, and out for speakers. If you already see 5.1 onboard you should find 2 output jacks for front, rear, and center.

If only one you may need to get an external non powered adapter to patch the 5.1 plugs on the speaker set into that seeing one stereo mini plug going to the laptop. The speakers themselves already see an amplifier of their own. The specifications for the 1720s and 1721s indicate on single headphone/audio out jack seen on those. http://www.dell.com/content/products/productdetails.aspx/inspnnb_172x?c=us&cs=22&l=en&s=dfh
 
That's an external module there likely seeing an ac adapter for power intended for taking the two channel audio output and snthesizing the 5.1 surround sound.

Here I have the opposite situation going from 5.1 jack on the sound card through the 5.1 Logitech 640x speakers out through the head phone jack on the center to cordless headphones! The mixer for the sound card has to be set at headphones unless simply playing some audio tracks with those unplugged.

That's geared for this type of application over trying to patch a 1/8" stereo mini plug at one end with 3 female plugs at the other for straight wire since that sees a preamp as well.
 
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