If you are plugging your headphone output of your phone or MP3 player directly into an input of your mixer using a 3.5mm TRS to 1/4" TRS jack cable you may notice the audio does not sound correct. This could be loss of vocals, bass sounding weak or general distortion. This is a phasing issue which is the result of supplying a mono input with a stereo signal and can easily be avoided using a specific cable type. The cable type you would need is a '3.5mm TRS jack to 2x TS 1/4" jack stereo breakout cable' below is an example of this cable so you know exactly what you are looking for:
You can then utilise one of the stereo channel inputs (shown below) on your mixer and your audio will play from your phone or mp3 as you would expect.