Why mp4 stored in S3 is not working in audio tag?

I have a rails app with an audio player that plays an audio file stored in S3. The player works locally, but not in production (Heroku). The linked audio file plays fine when pasted in the browser. I am not posting code because I don't know where the problem is (if it is a code problem) and besides it works locally.

Anyone give me a gut feeling about what is happening?