Build 1.20 (and actually even 1.18) of gstreamer plugins is TOO NEW for the version of gstreamer currently included in SFOS. Use older plugin versions (or upgrade gstreamer to a newer build…).
I’ve successfully used AMR codecs from 1.16 build of ugly codecs (gstreamer1-plugins-ugly-1.16.1-1.el8.aarch64.rpm), and 0.1.5-7 version of libopencore (opencore-amr-0.1.5-7.el8.aarch64.rpm). Such a combination works perfectly fine.