OpenShot is a simple open source video editor with support for Windows, Linux and Mac. Basic use cases include putting an image/video track and an audio track and make them play at the same time.
Shotcut is also a nice video editor that received a 4.5/5 stars review form TechRadar.
Other open source alternatives that are not as minimal and simple as these two may include Avidemux, Hitfilm and Movavi.
Windows Movie Maker is not open source but with it you can easily do the same. Development and official downloads have been discontinued, but the same features are offered in Microsoft Photos. Not open source either but Microsoft is progressively open sourcing its essential Windows app, like Calculator.