You have lots of options. For example:
1. Search with Yum:
a) The specific package is probably "libopencore-amrnb-dev"
b) You could try "yum list libopencore-amrnb-dev", or equivalent
<= No, I wasn't able to find it on my Centos.
but I didn't try very hard
c) You can Google for "yum install" syntax, or add additional
repositories to your "yum" search list
2. Search the Internet for the RPM
a) Go to
http://www.rpmfind.net
b) Search for "amrnb" and pick whatever looks best
3. Compile without "--enable-libopencore-amrnb"
4. Etc.