Sorry for not posting earlier on the subject of using Audio and Video for instruction. I tend to be more of a "lurker" than a poster.
Your description of going to "remembered" sections of a video describes exactly the problems inherent with working with both audio and video. How do go back to the important bits? Others in this forum discussion have also mentioned the ideal length of a video (around 5 to 10min.). However, from an editing and production standpoint, getting content into shortened format can require a significant amount of time.
As part of my SFU Masters thesis (The Audio Re-Searcher: examining the effects of audio note taking in a multi-media, web-based environment – see: https://theses.lib.sfu.ca/sites/all/files/public_copies/ETD6867_FZander_zanderfrank_finalthesis_pdf_18644.pdf ), I created an online tool for working with audio see: http://www.screencast.com/t/9W8HSKUnv91U
As it turns out, the tool (www.audioresearcher.org) also works great for annotating and bookmarking audio.
For a fun example of video bookmarking, see:
http://www.audioresearcher.org/default.aspx?Title=The Parrot Sketch&AudioFile=http://www.audioresearcher.org/upload/The Parrot Sketch.wmv