True...Let's look at it from a different angle then. If the music is in an unsupported file format (I have to say, I do not know which formats Google Player will play, as I use Poweramp which will play anything) say FLAC (lossless) etc...If there isn't a program on the phone that recognizes that file format, it will remain invisible. I know Poweramp plays ANYTHING in an audio format.
That said, Les, download the free version of poweramp and see if your files reappear. I just loaded 3 albums up to dropbox, (one FLAC, one MP4, one WMA) and downloaded them to my phone just to see what happened, and all three are fine. Maybe the extension is getting corrupted somehow...Dunno'. Without the phone in my paws, I can throw ideas out, but it is impossible to troubleshoot.