YouTube Audio Library

The Audio Library offers tracks and sound effects you can drop into your videos at no cost, filterable by genre, mood, instrument and duration, and most of it can be used in monetised videos. Some tracks carry an attribution requirement and the library flags which ones, expecting a credit line in your description.

The catch is that it is free and enormous, so everybody uses it. Certain tracks have appeared on so many videos that viewers recognise them instantly and read the whole thing as low effort. The clearance is also specific to YouTube, so a track cleared here is not automatically cleared for the same edit somewhere else.