I recall that the common merge is just that. And you need a better tool.
I see such tools noted at http://stackoverflow.com/questions/10437750/how-to-get-the-real-actual-duration-of-an-mp3-file-vbr-or-cbr-server-side along with why it's only so accurate.
I have been adding a number of audio mp3 files into one file. My issue is that it appears to only record the time of the large merged file as the time of the first individual mp3 file. So say the total merged files is say 15 minutes the file only has a time of say 1:30 minutes. (the time of the first file. ) This creates an issue in that I cannot pause and come back to any point greater than 1:30 minutes.
1. Change the total time of the mp3 file or
2. a merge program that adds the total of the mp3 files