I would like to have a better understanding of the "Copy To Devices Manually" Option in the Recordings section.
In my case, I am using BandHelper for leading worship at my church. Currently, I have loaded just over 250 songs each with its own PDF chord chart.
I grant access to my account for those who need to learn the songs the preceding week.
This week, I tried a little experiment to see just how practical it would be to add MP3 files to the songs, so I added a dozen or so MP3's, and in each case I selected the "Copy To Devices Manually" Option in the Recordings section. The reason I used the option is not because the individual MP3 are particularly large (they average about 5MB each), but because anyone who has Auto-Sync enabled (default setting) could theoretically end up with 250 MP3's (approximately 1.2GB) hammering their phones, and that wouldn't be a very considerate thing for me to do.
So, the desired scenario would be that if they DO want to hear a recording, they would click on the grayed out recording icon in the song,and it would be downloaded from the BandHelper account to their local device for playing.
For now, I would think my fallback position is to manage the "Copy To Devices Manually" Option for the songs we're singing on a week-to-week basis. I suspect, however, that if the MP3 was already present from Week 1, and then I checked the box, it would not be removed from their device, but at least its better than a 1.2GB surprise.
Copy To Devices Manually is useful if you do want the files on all the devices, but the files are too large to practically sync over the Internet, so you want to copy them directly to your devices via USB.
What you want in your situation is to turn off Settings > Account Sync > Auto-Download Files. Then the files will only download on-demand when you click them.