This isn’t the case anymore: [Paul Stoffregen] has just released his Teensy Audio Library, a library for the ARM Cortex M4 found in the Teensy 3 that does WAV playback and recording, synthesis ...
The BBC Sound Library offers 33,000 sound samples from a wide range of categories for non-commercial use. Freesound.org provides over 650,000 diverse sound samples under Creative Commons licenses ...