Here is my YouTube one.

The AES does presentations open to the public and archives them here. There is another by JJ that is going to be added soon on signal bandwidth.

For headphone design principles, you can search for the book "Headphone Fundamentals" by Carl Poldy. Toole's book covers headphones to some degree as well.

I also have a copy of "Loudspeaker and Headphone Handbook: Third Edition" that I managed to get cheap and use as a reference when I need to look something up. It doesn't have everything and I don't quite understand everything but it has a lot of info, lol.

Quick edit: Here is another neat YouTube channel. And two videos on the basic backbone of digital audio: 1 2

And intro to Fourier Transform.