The mention of "Behringer" brings to my mind a sad experience I had with that brand of mixer (I don't remember the model number now, since that episode with Behringer issues happened probably four years ago). My point here is that you should examine the output of that mixer to see if it is clean. I suggest connecting a good pair of headphones to the headphone jack on the mixer to see if you get clean audio out of it when you sing.
The model I had was one of their larger mixers with the digital effects processor built in. The first one that I received from the company worked only a day or so, then the effects processor began to make various weird squeaks, white noise, and other strange sounds. I got a replacement for that mixer promptly. I was able to use that unit for several months, then it began to operate erratically. Fortunately, I had purchased a protection plan when I bought it, so I received reimbursement for the unit. The odd thing is that I am running a few of the smaller, simpler Behringer mixers on the stereo installation for my wife and one for me; they have worked beautifully for several years. It was apparently the digital effects processor which "bit" me on both units.
I read somewhere on the Internet back then that the company had at least for a time a problem with "cold-solder connections". Without getting too technical here, a "cold-solder joint" happens when a connection is not heated quite enough for the solder to flow and make a good bond between the two items which are supposed to be joined. That connection might work for some time - maybe even a few months - but it is likely to fail eventually. Once that happens, the unit is likely to malfunction in one way or another.
If everything else you have checked provides clean output, then the mixer should be suspected as a possible source of the problem. Again, I don't know how old your mixer is, but its performance should also be checked in your troubleshooting process.