Why are my customers having trouble downloading mp3s on their phones?

[AC Question]

I have a business where I sell mp3 files. Lately a lot of customers are having issues where they say that the files won't play. When I get a complaint, I then email the files to them as attachments, which seems to work fine, but I like it when customers can get their products immediately, without having to wait until I get to my email.

I tried doing a test purchase myself on my own Android phone, and it worked fine. Any ideas about what is going on and how I can address it?