I think the clue to your problem lies in that little phrase "from my mobile". It seems that almost all mobile devices produce files at "variable bit rate" and this seems to give all non-linear video editing programs, like MEP, considerable difficulties! MEP 17 is now quite "old"; I don't know whether a later version can handle such files without difficulty.
To expand Jeff's answer re mobile phone video - not only are they VBR as Jeff has said, the audio saple rates are often not the 'standard' 48kHz but range from 22 - 44.1 kHz depending on the mobile manufacturer and it is the difference in the audio sample rate which can cause the loss of sync you are seeing.
IMHO you should use the the right tool for the job, ie a proper video camera for video and leave the mobile phone to do what the job its name says.
I fully agree with John about using the right tool but appreciate that you probably don't always have your video cam with you and that UFO isn't going to wait for you to run home to get it!
This is an instance where I think it is appropriate to suggest your trying some other product just to make a bit rate / audio sample rate compatibly copy which you can then more easily use in MEP -- which is IMHO the best tool for editing.
Does the footage play smoothly in the MEP preview screen or is it perhaps that the picture is jerky and the sound smooth that makes it look like it is out of sync. In other words does the edited material play back out of sync on the phone or other devices after exporting to the correct format / profile?