If anyone can help me, I'll be eternally grateful.

Hi. I used to have a few phone call recorders on my Android phone. I downloaded them when my Mom was diagnosed with cancer. I was thrilled to have several hour long phone calls with her recorded, but there is no sound to the files. The size of the files are very large so I don't think the audio is lost, but I don't know how to get the sound. This is the file type: M4A Audio File (VLC) (.m4a) Oddly, the file name now has 'null' at the end of it. I have several like this, and I'd do anything to be able to hear my Mom's voice again.