When you insert an mp3 source file into an edit, and then immediatly render that edit. The audio content from the source file will be missing from the rendered output.
However, it you wait 1500ms to start rendering after inserting the mp3, then the rendered file will sound as expected.
This is due to a bug in in tracktion_engine caused by the custom "width" rack that cybr inserts onto every track.
See this thread on the juce forum for details: https://forum.juce.com/t/inconsistencies-when-rendering-an-edit-after-inserting-mp3-files/46110