Emms is free software, but some of the file formats it can play carry restrictions, they are proprietary file formats. Proprietary software companies are pushing out audio and video formats which restrict when, where and how you can play them, and restrict developers from writing free software which interacts with them.
Restrictive file formats put the corporate bottom-line before the public interest.
Fortunately there are alternatives like Ogg. Ogg is a professional grade multimedia format. Ogg Vorbis is the compressed audio format (like MP3), and Ogg Theora is the video format. For more information, go to http://www.xiph.org/.
If you want to transcode audio into a lossless format, you can try FLAC (Free Lossless Audio Codec). FLAC stands out as the fastest and most widely supported lossless audio codec, and the only one that at once is non-proprietary, is unencumbered by patents and has the source code for a reference implementation freely available. For more information about FLAC, go to http://flac.sourceforge.net/.