iOS does not have a user filesystem.
1. Get a DropBox account (FREE)
and the iPhone/iPad Universal app. It registers in the OS, and when you download files, you can transfer them from Safari and other apps into DropBox. From there you can playback.
2. iUnarchive (FREE)
Can unzip packages and load files, and send the unzipped package to DropBox.
First of all, as suggested above, you will need Dropbox and an account. Get that done first.
Whence Dropbox is installed and setup, navigate to the mp3 file, tap and hold the link and a pop up will display list of options, including save to Dropbox.
Another method is to install iCab mobile: https://itunes.apple.com/us/app/icab-mobile-web-browser/id308111628?mt=8
iCab mobile will let you download mp3 files directly onto your iPad and store them there. Additionally, you can play back mp3 files within iCab.
Further, iCab offers Dropbox integration, which brings us back to the top.