There is no way how a pictures like PNGs could be converted to MP4 audio, which means direct png to mp4 conversion is unlikely. However, MPEG-4 (.mp4) is a container for multimedia content, including video or audio and may also contain images.
You can find presentation programs with which users my prepare videos that are like slideshows of images with audio playing in background, similar to what you can see on YouTube. You can find more about this subject if you google "picture to video" search string.