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1) There should be a way to get both the sound + mic to work with your existing setup (just about every desktop has a sound and a mic input), but I’m a little unclear as to what you’re describing in your setup. I’ll try just explaining from scratch in case I misread/misinterpreted something you wrote. Most onboard sound (as well as most sound cards) have at the very least a green and a pink jack which you should see at the back of the computer. Sometimes they’re on a front panel as well, but if that’s the case you may have to try both the front and the back because depending on how it’s been set up, one may work and one may not. Green = sound – Pink = Mic.

Typically, a headset has 2 plugs – one for sound, the other for the mic. Generally, you can leave the headset’s mic plugged in all the time (to the pink port), but if you want sound coming out of the headset, you’ll have to UNPLUG the speakers and plug the headset’s sound into the green port.

This obviously gets annoying if you switch between the speakers/headset often, so an option is to buy a 3.5mm extension cable (you can get them from Monoprice for quite cheap), and then have the regular speakers plugged into green, headset mic plugged into pink, and the headset sound plugged into the extension cable which you run towards the speakers. When you want sound from the headset, you then plug that extension cable into the 3.5mm jack on the front of your speakers – the speakers will stop putting out sound and the headset will start putting out sound. The mic will always work.

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2) Getting to the USB question (if you can’t the above to work), the answer is YES it will allow the sound and mic to work at the same time.
2a) If you use a USB Microphone, your existing speakers should continue to work fine, and you’ll just get the added benefit of the mic. If you don’t really need headphones (don’t mind the sound always coming from your main speakers, or if you WANT the sound to always come from your main speakers), this is usually the BEST option.
2b) If you use a USB Headset (microphone + sound), you’ll get BOTH mic/sound out of the headset. Note that you will have to plug the headset in BEFORE you start the program/game you want to use it in. Also, the sound to your speakers won’t work while the headset is plugged in because USB headset’s typically change the default sound device to themselves. When you unplug the USB headset, sound to the speakers should return to normal (although again you’ll probably have to restart the game/program you’re in).

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If anything’s unclear, or if your setup is quite different, reply back with some more details and I’ll try to provide you with a specific solution rather than the general one.

thanks