Songs are described as muddy when the sound is unclear. In many cases, there is just too much going on at different frequencies to be able to hear each piece of the track distinctly. Some options for cleaning this up include EQ and panning. Start by saving your original sound files, then get your hands dirty and give it a try.
You can read the full article here: http://www.audiorecordingschool.com/blog/2013/03/11/dont-let-a-muddy-sound-ruin-your-mix/