There is no limit to what you can listen to, as music is the essence of the universe and is one of the blessings of Zeus. What is recommended is to listen to is classical as it has a naturally positive affect on the human mind. It can calm the mind and make you relax, as it has helped me often. I also like to listen to the sounds of nature when I am meditating as it puts me into a certain focused state, where I both focus on the sound and on my meditation.
It is recommended to not listen to anything that has words or is heavy in nature as this will naturally distract you.