antennas are used to pick up radio frequencies. Antennas are conductive material, that is not grounded, if it is grounded, then the picked up signal is routed to ground. That's what a shield does on a mic wire. If you were to take apart a mic wire the first layer of material undernieth the outside insulation is a bare wire that surrounds the entire two signal wires. This is the shield, any outside frequencies like a country station that hit the wire should be sent to ground. This is your most likely problem, some how the wire might be acting like an antenna. This might be cause you're wire isn't good, or your house isn't grounded properly. try these in this order.
1. unplug the mic cable while you have your headphones on and you can hear the radio,
2. If it goes away, try another cable. if it's still there
3. here's where some money might be invovled, I would say that your house doesn't have good grounding before I'd say it's a Mackie board. Here's my disclaimer, "don't mess with elelctrical wiring in your house unless you know what you are doing"!!!!! I'm an Electrician, and I've seen lots of stupid accidents, mostly from me....lol. Anyway,
4. Go to Lowes, or Home Depot or whatever, and get four things, about 20-30 feet of ground wire (ask them they'll know what to give you) and a 8 ft. copper rod. and a receptical grounding adapter, it has two prong holes and a little green metal tab to attach a ground wire to. It's made to give a ground to a non-grounded outlet that has only two prongs. And a 1/4" hose clamp. By the way the round uhole in an outlet is for the ground, don't get offended, just making sure. Ok where was I, oh yeah, get those things, not sure how much it'll be,
5. Drive the copper rod into the ground outside your house with a big hammer next to the window of your studio room, or close to a crawl space. Attach the grounding wire to the copper rod with the hose clamp after you drive it fully into the ground with about 6-8" still sticking up.
6. run the ground wire into your room, this is up to you how you do it.
7. Put the ground wire on the grounding adapter tab (it's green) and plug it into the outlet. Then plug your mixer into the adapter. The grounding rod outside is a guarantee that your don't have a bad ground. If this doesn't take care of it, then I don't know man, it's probly your mixer. I've solved many, many radio problems with this method, so give it a try, it's about the cheapest.
Hope this helps,