Decrypt and audio format and player

I have an android application that plays video files. The video files are inform of 2 files. 1. One file is an mp4 format which has the video but without audio. this mp4 is playable in standard video players. It will play the video, but with no audio. 2. Second file is a propriety format .eg abcdef . The second file is not recognised by any media player. The folder that has the video files, has 2 files for each video. e.g [login to view URL] and [login to view URL] . [login to view URL] containing the video with no audio and [login to view URL] containing the audio. *.abcdef being a propriety or custom format. At the end the code for each of the mp4 files I checked, it is indicated that "freeIsoMedia File Produced with GPAC 0.4.5" So, I assume the original videos were encoded using GPAC to produce the 2 files. A seperate video and audio file. Task: 1. Analyse the *.mp4 file and *.abcdef file to determine which encoding/encryption has been used. 2. Analyse the media player in the android app to determine what features has been enhanced to enable it to read and play the 2 files simultaneously. Also determine how it is able to read the *.adcdef audio format. I dont have the source code for the app. So you would have to reverse engineer the apk(decompile) 3. Determine a way to take a standard video file, then produce the 2 files from the video. An *.mp4 and *.abcdef file that will play in the android app. This will require knowledge on GPAC.