Exported file have no audio

Thanks for this. Just switched to a new audio interface and I didn't realize it had switched from Mix/Master. Thank you!

There's a good chance the audio you are not hearing has been recorded as a stereo track but with a mono signal which is out of phase. Sound will be heard on any device playing back in stereo but will not be heard on and mono playback device (like a phone). This problem is often caused by using a balanced mono microphone (XLR connector), which has three wires, pluged into a 3.5mm stereo jack, which also has three wires, but is two channels unbalanced. 

Track 7 is the only one playing after export it has the music

• I have the same problem the audio is not playing is some devices ( phones) I only heard the additional sound not recorded on set . But all the dialogue are mute . When I play the video on my pc the sound is there . The tracks are on mix (output) expected for the mix track which is on (default stereo output) . 

Haven't seen the mixer view, but I can tell what's wrong anyway - with the track you've got exposed, its output is being fed to your system speaker, rather than 'Mix' (or Master) - which is where it should be sent if you want it to be included. The idea is that you send all of your tracks to 'Mix', and only the Master output to your speakers.

 Should be noted that wav can be exported with sound but mp3

Buffer size has nothing to do with exporting, although that does seem a little small, I must say. None of the hardware settings affect this either - it's purely a software function. Generally what you are experiencing is directly related to track and mixer settings in Multitrack view - screengrabs of your session and mixer settings, and all of the subscreens in the Export panel might give a bit of a clue...