When we export our project to MP4 one of the audio tracks stops playing part way through

We are trying to export a 12 minute film from premier as an mp4.  The project has 3 audio tracks and 3 video tracks, so not huge.  We exported it as an MPEG with no problems at all, but when we tried to make an MP4 version, Audio 2 stopped playing about 3 minutes in.  "Blooming glitch!" we thought, "let's just re-export."  With the second MP4 we generated, Audio 2 stopped playing 6 minutes in.  Can anyone tell me what's going wrong? 

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