after mixing down into m4a, my track is actually 3 mins or so, but the track shows that it is 9 mins… last 6 mins or so just silent theres nothing, i dont get it. please help, many thanx! 
Hello,
You should check your locators.
Make sure that they are highlighting only the audio that you want to export.
EDITED: It will also be good if you could send us the project, in case it is a bug we will be able to reproduce it and fix it.
Best regards,
GN
Guillermo… I can send you mine if you let me know how. Here it’s 2018, 4 years after the OP, and I’m having this same issue! PLEASE HELP!!!