need a splitter or usb device to be able to use mics and line out at the same time for live stream of meeting.

I have a Dell laptop that I am using at a hotel to stream a live meeting.  I have wireless mics that work well. I plug them into the headphone/mic/line jack on the computer and all is good. The hotel that the meeting will be at needs that port for audio out (the plug is a normal headphone jack into my computer and the other end I believe is XLR into their old audio system)  from the laptop (like in a powerpoint video) to be heard over the house speakers in the hotel. The remote attendees can hear this audio as well through the gotomeeting software I am using. The problem is I lose my mics. I have a splitter that is a speaker/headphone plug on one side and a mic plug on the other.  This does not work.  I have also purchased a USB device that has 2 ports like the splitter. This also does not work. Must be something with my sound card for this particular laptop?  Question is this:  I need some sort of a setup that I can use my wireless mics AND still be able to plug in the hotel cable into my computer for audio out for the present attendees to hear.  Is there some sort of a mixer that plugs into USB that could accomplish this?