If your mp3's are in the music folder on your computer with Zune, just associate the mp3's with Zune and sync them into your phone. That is all you have to do. If associated with Windows Media Player, then Zune will recognize them.
I have found a MP3 downloader app for windows 8 in the store. tried dowloading the file, i was able to download. the tricking part here is, after downlading the mp3 file i couldn't found where it was stored. i am worried of my GB space here as i am no more using this file, want to delete this completly from my phone. . I also tried downloading it for the second time and i could see a error saying this file already exist in my mobile.
The same was repeated when i have downloaded a video from you tube using youtube downloader app that i ve found in the store.