videos in AudioSet
A list of about a million ten-second YouTube clips, each labelled with the sounds in it, that Google published in 2017. Labs use it to train and test general audio and music models.
Including "Essential Techniques for Mandolin" with Virtuoso Chris Thile (Nickel Creek, Punch Brothers), "Basic Techniques of the Classical Mandolin" Caterina Lichtenberg, Get Rolling - introduction to Bluegrass Banjo by Pete Wernick