The Living Thing / Notebooks :

(Audio) sample libraries

A particular type of free content, or a particular subclass of data sets, for making your own music.

Audio, General

Drum samples

Who knows which of these are legal, but there is an endless churn of drum sample over on Reddit.

Impulse Response libraries

Libraries of reverbs, that may be convolved with a signal to make it sound like it was recorded somewhere else - e.g. record a sound in a church then play back other sounds as if they were played in the church. Aside: there is an interesting deconvolution problem in making these things, and possibly an even more interesting one in synthesising them.

Sources, via, e.g. sidebrain:

And ultimately anything can be treated as an impulse response; although you might have to do some fancy footwork to make it sound ok - see above re: deconvolution.