You can transcode your file into a format Rush supports by using Adobe Media Encoder, or if you're using the free trial of Rush, I've heard good things about the third-party transcoder, Handbrake. You can see their options here: https://handbrake.fr/
I ran into a similar problem importing a sound file that wouldn't work. I then opened it in Quicktime then exported it. The resulting file did play properly in Rush. I also have Handbrake but didn't need to use it in this instance.