Is this video file directly from a camcorder? If so, you'd be best to shoot any video that includes 5.1 audio as an AVCHD file rather than an MP4. Premiere Elements will work perfectly with AVCHD 5.1 video. I'm not sure it can do this with MP4s however.
What is the specific camera that is recording your video with 5.1 channel audio? What is the audio AAC 5.1 channel?
I comment here because of what you wrote and what has not been commented on by you....
When I import my mp4 file onto PE, the 5.1 audio will only play the background effects, music and not the spoken audio.
How are you getting the spoken audio into the file from the camera. I would expect it to be part of what the camera is recording as the camera records video and audio at the same time. Are you mixing the narration into the file with audio from an external mic somehow in the camera? If so, what are the details of that.
There should be no problem importing an AVCHD.mp4 file with its AAC 5.1 channel audio into the Premiere Elements project described for use with 5.1 channel audio. I have been there and done that.
We will be watching for further developments.
The file isn't directly ripped from a camcorder.
The file isn't recorded from a camcorder, it was given to me as a full mp4 file. It plays perfectly fine on vlc and any other video player but will not play the full sounds while it is in PE.
If it would not invade your privacy, you could send a small sample of the file by posting a link to it in DropBox or Google Drive in a thread reply. I do not use DropBox, but I do use Google Drive. Consequently, I can give you a step by step for Google Drive.
One of the first things that I would do is to try to get the audio from this file into Audacity to take a good look at its channels. But, I will offer to give the whole file a workout in Premiere Elements Windows if you decide to send a sample of the problem file. And, I will report the results in your thread.
Are your that you are dealing with 5.1 channel audio and not 4 channel audio?
Another alternative is to get more information about this file's origins and properties from the sender of the file.
The fact that this is not an AVCHD 5.1 camcorder is what makes all of the difference. It's not clear how this audio was output, but it is apparently not done to AVCHD 5.1 standards.
We will need to analyze the video to figure out what is wrong with the file and how to fix it.