I haven't used the in built mic, but I'm guessing it's designed for skype calls. ie. low quality/bandwidth and designed to pick up someone facing the display. It might even have some noise cancelling to try and mute sounds not coming from directly in front of the screen, but I'm just speculating.
Have you checked the mic levels in the control panel? Right click on the speaker in the desktop notification area>Recording devices>Select the internal mic>Properties>Levels
There is a dB boost there as well. but it might make things sound distorted if they are too close and will raise your noise floor.
For a meeting, best bet would be to get an omnidirectional USB mic. Sit it in the middle of the table and it should pick up everyone fairly evenly (provided they are speaking at about the same volume).
If you want to be a bit more discreet or portable, then something small like the Samson USB GoMic might be good enough.
I don't think the 1/8" audio jack is a combo headphone/mic, but if it is, something like the iRig MicCast might work. I know it works well on Android devices with combo audio jacks.