I think it's a really good idea if you made it easy enough to record and reply. The majority of your value will be based around those specific interactions. (Video hosting is pretty much solved.) I would focus a lot of your effort on the UI/UX end of it. If you're green on UI/UX, read: http://designingfortheweb.co.uk/book/. Alternatively, hire someone who rocks.
Additionally, I'm not certain online music teachers would be willing to pay for such a product. Have you talked to any yet? Make some interactive mockups (Balsamiq or Keynotopia) and visit a few schools. Try to find people who are already sharing tutorials online and ask them.
Thanks Mike, that's helped me see where I need to focus my attention.
I was looking at direct webcam recording, but was going to add that later. But given your comments I think that is a must now, to enable quick and seamless video comments and feedback.
Regarding music teachers paying, I mentioned in the other thread to Matt that I may be thinking more towards making it free for teachers, charging students per course, and taking a percentage.
Re mockups, I am creating html mocks, but I'll look at the quick prototyping mockups so I can pitch to various music schools.
Additionally, I'm not certain online music teachers would be willing to pay for such a product. Have you talked to any yet? Make some interactive mockups (Balsamiq or Keynotopia) and visit a few schools. Try to find people who are already sharing tutorials online and ask them.