You will need an MP3 Shield of some kind to do the decode. Google can help there.Since you talk about a browser you will also need some kind of network access. That can be an Ethernet Shield if you have an Ethernet outlet handy. The Ethernet Shield also has an SD (or Micro-SD) car slot for file storage. If you need to use a USB Thumb Drive instead of an SD card you will need something like a USB Host Shield.Once you add all this additional hardware you might be better off getting something like the DUE or even the RaspberryPie.
(...) And if you are using an MP3 shield, you have complete control. You have to load the file off the SD card and stream it to the MP3 decoder, so you can open whatever file you want. (...)