Agreed. I've added a subsection on keeping track of sources. Essentially, I recommend using a spreadsheet.
When remixing media files (e.g. images) it can be useful to store them in folders "CC-BY", "CC-BY-SA", "GNU FDL", "PD". Then each folder could include a spreadsheet with attributions. At first, just the URL of the source for reference, and when actually using an image, obtain the correct attribution from the URL (add it to the spreadsheet, and include it in the OER's list of attributions).