Quick help with church sound

Hey friends --

I'm a pastor and we don't have a smart sound/tech guy so it gets thrown to me.  I don't really know much.

The last couple weeks, my wireless mic has been screaming and completely unusable.  Even when it's in the back where there are absolutely no speakers or monitors or anything, even if the fader's completely down and the channel's muted, the light it's channel of the board that indicates that it's picking up a signal is strong, and of course if it's unmuted or slid up there's an immediate terrible sound.  Does anybody know what's happening or what could fix it?  It's a cheap Nady system, do we just need to upgrade?  Although it's worked fine for a couple years and this just started recently, I don't know! 

Unrelated but since I'm posting -- I'm also trying to connect our computer and an external mp3 player into the board.  In the past we've had to use the same cable (which means switching back and forth between the two, which is a hassle), and has been in through the "tape in" which is a separate input from the "normal" channels with XLR and 1/4 inputs, faders, eq, etc.  So I got some adapters and cables and tried to plug these into the dual 1/4" inputs on the board, but no signal comes through at all.  I don't understand -- they're rca cables, is that a problem?  If it matters, the board is a Mackie 1604 VLZ Pro, which I think it a pretty typical thing. 

Anyway, if anybody has any help or tips, I'd be very appreciative.  I'm usually able to figure things out with some work and thought, but these have me flummoxed.  Thank you!

Usually on the back of the receiver box for the microphone, there is a button or switch that reverses the polarity. That might be the problem ?  Also there could be an issue with improper grounding in the plugs you're using for power.  A couple of possibilities ?