According to a senior level Apple Support Rep, Apple has coded their OS to specifically prevent any audio file that does not have DRM protection, to be downloaded. This means Apple products will force a file to stream as opposed to downloading the entire file. Finding an official statement from Apple is difficult so the closest I could get was talking to a senior rep. You can, of course, call apple for free support with ANY apple product (including I-tunes on windows lol) by calling 1-800 My Apple. Most likely you can get additional confirmation there.
I actually created a site where if you click on a link with a windows or android device, it forces the download. This doesn't work on Apple devices, it still will only stream. The only real fix for this is to offer the file as a compressed file. (ie. zip) then they could download the compressed file but then they would have to open the .zip up and extract the audio.
Ironically, I'm currently looking into how I can use PHP to "zip" a file up on the server side per client request and then deliver them the file in the compressed format. I stumbled across this post because I was attempting to see if there was a way to fool a system into believing a file was in fact protected when it was not.
Hope this helps.