This page aims to give you an overview of some possible sources that you can use in finding music, sound effects, samples etc., with re-use licenses or other clear guidelines on what is allowed with the source material.
There are some websites where you can find and use music with permission of the composer, for example:
Audionautix (info about this source (in Dutch): https://rsnijders.info/vakblog/2017/11/08/muziek-zoeken-audionautix/ )
Free Music Archive (info about this source (in Dutch): https://rsnijders.info/vakblog/2018/02/01/muziek-zoeken-free-music-archive/ )