Won't play my MP3 audio file so I can edit it.

The usual (like 99.0% of the time) cause of a file loading proper but then refusing to play is that Audition has lost control of your sound card/audio interface.  Particularly if you use the generic Windows drivers (or the Apple equivalent) other software can be deemed more important and take priority for use of the audio system.

Your first step would be to go to Edit/Preferences/Audio Hardware and check that the proper audio driver and device is shown. Even if it looks right, reselect things and apply it again.

If this doesn't work, go to the Windows Audio Control panel and set the device you need as the default device then repeat you visit to Audition's preferences.