I've been downloading mp3 audio files for last 2 years as part of school assignments. All of a sudden they start to download with a .null extension and I'm not able to play or listen to them. Any help would be appreciated!
Well if they say null then they likely didn't download correctly. There may be a problem with the location you are downloading them through. So I would check that. You can look at the name of the file before you download it and make sure it has the right extension. If it doesn't, then you need to let the location you are getting them from know there is a problem. If, however, they are named correctly before download, then there may be a problem either with whatever app you are using for downloading, or with the phone itself. A simple restart (turning off and back on) the phone may resolve your problem. I would start there, should the file names be correct, as mentioned above.
You might also want to try downloading the files through another device, like a computer, and see if they turn out fine or not. To test the issue.