Here is a small snippet, showing how to answer a call and play audio from your opensips config using the opensips b2b_entities module.

if (is_method("INVITE") && !has_totag()) {
        xlog("[$fU $tU $ci $rm] initiating ua_session_server_init\n");

        ua_session_server_init($var(b2b_key), "ivr");

        xlog("[$fU $tU $ci $rm] running rtpengine_answer\n");

        rtpengine_answer();

        xlog("[$fU $tU $ci $rm] ua_session_reply with '200 OK'\n");

        ua_session_reply($var(b2b_key), "INVITE", 200, "OK", $rb);

        exit;
    }