Issue with AMR file structure
Actually, if it's got 160 samples every 20ms, you get a sampling rate of 8000 (you're missing a zero), which is the standard for over-the-phone voice communication. from a Google search, I couldn't find any AMR-NB specification that uses a different sampling rate.

Also, I found this line here pretty useful:


  For AMR-NB, only an “8000” sample rate and one of the following
  bitrates are valid:
  4.75k, 5.15k, 5.9k, 6.7k, 7.4k, 7.95k, 10.2k or 12.2k


Wikipedia also mentions that AMR-NB uses a 8000 Hz sampling rate, you can be pretty sure that this is the case all across the board.