I don't know what FFmpeg version Steve has but I'm trying now with git ffmpeg.
I couldn't find out how to configure FFmpeg for a native AMR*b encoder/decoder
or even if there is one, so I got opencore with the dev package and I
configured FFmpeg thus

./configure --enable-shared --enable-libgsm --enable-libspeex
--enable-libtheora --disable-encoder=aac --enable-version3
--enable-libopencore-amrnb --enable-libopencore-amrwb

Am I right thinking that config enables libfaac but keeps the native aac
decoder? Couldn't find any docs about it. 

FFmpeg built and installed fine and I did ldconfig after that. Pkg-config seems
to show Audacity should link to this when I compile it using system ffmpeg. But
building Audacity unicode release against your v6 patch  I ran into:  


 FFmpeg.cpp -o FFmpeg.o
In file included from FFmpeg.cpp:21:
FFmpeg.h:308: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:512: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:512: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:518: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:518: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:600: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:600: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:612: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:612: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:618: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:618: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:624: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:624: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:630: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.h:630: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.cpp:258: error: invalid conversion from ‘int (*)(URLContext*, unsigned
char*, int)’ to ‘int (*)(URLContext*, const unsigned char*, int)’
FFmpeg.cpp:261: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:342)
FFmpeg.cpp: In function ‘int ufile_fopen_input(AVFormatContext**, wxString&)’:
FFmpeg.cpp:299: warning: ‘ByteIOContext’ is deprecated (declared at
/usr/local/include/libavformat/avio.h:366)
FFmpeg.cpp:328: warning: ‘int get_buffer(ByteIOContext*, unsigned char*, int)’
is deprecated (declared at FFmpeg.h:518)
FFmpeg.cpp:328: warning: ‘int get_buffer(ByteIOContext*, unsigned char*, int)’
is deprecated (declared at FFmpeg.h:518)
FFmpeg.cpp:341: warning: ‘int64_t url_fseek(ByteIOContext*, int64_t, int)’ is
deprecated (declared at FFmpeg.h:618)
FFmpeg.cpp:341: warning: ‘int64_t url_fseek(ByteIOContext*, int64_t, int)’ is
deprecated (declared at FFmpeg.h:618)
FFmpeg.cpp:401: warning: ‘int url_fclose(ByteIOContext*)’ is deprecated
(declared at FFmpeg.h:624)
FFmpeg.cpp:401: warning: ‘int url_fclose(ByteIOContext*)’ is deprecated
(declared at FFmpeg.h:624)
FFmpeg.cpp: In member function ‘bool FFmpegLibs::InitLibs(wxString, bool)’:
FFmpeg.cpp:914: warning: ‘int av_register_protocol(URLProtocol*)’ is deprecated
(declared at FFmpeg.h:529)
FFmpeg.cpp:914: warning: ‘int av_register_protocol(URLProtocol*)’ is deprecated
(declared at FFmpeg.h:529)
make[1]: *** [FFmpeg.o] Error 1
make[1]: Leaving directory `/home/gale/audacityc/src'

I won't be trying v7 for a while so if you are around please let me know what
you think about the above  error.