In libao-1.1.0, the function
char* ao_file_extension(int driver_id) is still defined in the header file (
ao/ao.h), and it is documented (http://xiph.org/ao/doc/ao_file_extension.html), however the function itself is not present in the library. You can check it by running
nm -D -C --defined-only libao.so :
0000000000000000 A LIBAO4_1.1.0 0000000000003440 T ao_append_global_option 0000000000003390 T ao_append_option 0000000000003ab0 T ao_close 0000000000003ba0 T ao_default_driver_id 0000000000003b40 T ao_driver_id 0000000000003db0 T ao_driver_info 0000000000003e10 T ao_driver_info_list 0000000000003460 T ao_free_options 0000000000002fa0 T ao_initialize 0000000000003e20 T ao_is_big_endian 0000000000004bc0 T ao_open_file 0000000000004cb0 T ao_open_live 0000000000003550 T ao_play 00000000000034a0 T ao_shutdown
So, either way there is a problem: either there is a problem in the code, or the documentation and header should be updated.