From your .3gp example, it describes its properties as follows (using KMP Player):
C:\Documents and Settings\DEVELOPER\Desktop\example.3gp
General
Complete name : C:\Documents and Settings\DEVELOPER\Desktop\example.3gp
Format : MPEG-4
Format profile : 3GPP Media Release 4
Codec ID : 3gp4
File size : 5.23 KiB
Duration : 3s 460ms
Overall bit rate : 12.4 Kbps
Audio #1
ID : 1
Format : AMR
Format/Info : Adaptive Multi-Rate
Format profile : Narrow band
Codec ID : samr
Duration : 3s 460ms
Bit rate mode : Constant
Bit rate : 5 200 bps
Channel(s) : 1 channel
Sampling rate : 8 000 Hz
Resolution : 16 bits
Stream size : 2.20 KiB (42%)
Title : SoundHandle
Writing library :
libVLC/VLC VLCJ, . , libvLC/VLC GPL AMR, 3GP- - ( ):
AMR , VLC FFmpeg AMR . , AMR VLC.
, http://mailman.videolan.org/pipermail/vlc-devel/2011-February/078807.html, :
AMR libavformat libavcodec FFmpeg, VideoLAN.
, AMR , AMR:
http://mailman.videolan.org/pipermail/vlc-devel/2011-February/078814.html