Here is a list of all files with brief descriptions:

include
osmocom
codec
codec.h
ecu.h
gsm610_bits.h
src
codec
ecu.c
ecu_fr.c
ecu_fr_old.c
gsm610.c	GSM 06.10 - GSM FR codec
gsm620.c	GSM 06.20 - GSM HR codec
gsm660.c	GSM 06.60 - GSM EFR Codec
gsm690.c	GSM 06.90 - GSM AMR Codec
hr_sid_class.c