The Aux-In on the ProTraXX and TourTraXX allow you to route a stereo line-level audio input to any of the TraXX's audio outputs. To route the audio, you issue a command that tells the TraXX to start playing the Aux-In on a particular channel, channel pair, or channel group. To stop the Aux-In playback, you send a stop command to that channel.
If an MP3 is playing on a channel and you issue a command to route the Aux-In to that channel, the MP3 playback will be stopped and the Aux-In audio will replace it.
See the user guide of your TraXX product for the full details of the Route Aux-In and Stop commands.