I think they may be m4a. Even if it's mp3, they are both 2 channel formats.
You can go from a higher bit rate to lower bit rate by losing some quality but can't go the other way. Similarly you can't create 6 channel sound from only 2 channels. Even if you did, it would not be true AC3 6 channel, with the other 4 channels simply having identical content.
I have only seen true AC3 6 channel tracks extracted from original DVDs. And possibly from Blu rays now.