Wrong length displayed for audio file So I have an audio book in a single mp3 file that's over 5 hours long. On my computer all players display it's length correctly, however on my phone Smart AudioBook Player and the Play Music app both says that it's only 11 minutes long. Rockbox picks up the length correctly.

What could be the problem and how can I fix it without using a different app?
Its most likely a VBR mp3.  as a DJ we have seen this a lot, The VBR is never detected correctly as the length of the playback is not proportional to the length of the file. 
With DJ software we have to prescan the files to determine their real length, obviously your application is only expecting FBR files and not VBR.
You could put it through something like Audacity on a PC and convert it to Fixed Bit Rate. but that is you only real solution, I think.