Running a 1/8" stereo mini plug out of the headphone jack on the back of the computer into the Line In on the mixer. I'm using a stereo mini into a stereo quarter inch adapter. When we play back some audio we don't get the full frequency range and in some YouTube videos we get decent background music but the audio is really garbled and distorted. Unintelligible. When I pull the 1/4" plug part way out I get full range and the audio is corrected. We are running into one channel for each of two computers. Any ideas?