The reason why you're experiencing this issue is because Apple don't want you putting in all your songs into another library (so they get all the money!). The way they have done that is by saving it in a format that only iTunes can read.
You could go through each song manually and convert them to MP3s (by going into the Options panel on iTunes, and changing the encoder to MP3, and then right-clicking each song and choosing "Convert Song to MP3").
Good luck!
