Streaming audio / mp3 files with 300k users [iOS]

We developed an app were users can choose a sound file and do several things with it like, record video, create funny effects. In the beginning we didn't have any problems with our servers, but since we have more then 300k users our server is getting very slow. We have a CentOs server and we only use a folder that has mp3 files in it.

In this app when a user chooses the mp3 it copies the file in to the documents folder, so the files are beeing 'cached'. Is there any solution to provide more than 300k users mp3 files? Because it's causing a lot of traffic.

Regards