fix: Use Opus in the CBR mode

A diff sent by an anonymous IRC user.

toxav/audio.c

@@ -377,6 +377,21 @@ static OpusEncoder *create_audio_encoder(const Logger *log, uint32_t bit_rate, u
         goto FAILURE;
     }
 
+    /*
+     * The libopus library defaults to VBR, which is unsafe in any VoIP environment
+     * (see for example doi:10.1109/SP.2011.34). Switching to CBR very slightly
+     * decreases audio quality at lower bitrates.
+     *
+     * Parameters:
+     *  `[in]`    `x`   `opus_int32`: Whether to use VBR mode, 1 (VBR) is default
+     */
+    status = opus_encoder_ctl(rc, OPUS_SET_VBR(0));
+
+    if (status != OPUS_OK) {
+        LOGGER_ERROR(log, "Error while setting encoder ctl: %s", opus_strerror(status));
+        goto FAILURE;
+    }
+
     /*
      * Configures the encoder's use of inband forward error correction.
      * Note: