I think I've found the memory leak, introduced in https://git.gnome.org/browse/rhythmbox/commit/?id=165a82e2cbeb87ba134e0caff2f588cebc51f7c4

static gboolean
impl_get_missing_plugins (RBEncoder *encoder,
			  GstEncodingProfile *profile,
			  char ***details,
			  char ***descriptions)
{
	GList *messages = NULL;
	GstElement *encodebin;
	GstElement *enc;
	GstMessage *message;
	GstBus *bus;
	GstPad *pad;
	gboolean ret;

	ret = FALSE;
	rb_debug ("trying to check profile %s for missing plugins", gst_encoding_profile_get_name (profile));

	encodebin = gst_element_factory_make ("encodebin", NULL);
	if (encodebin == NULL) {
		g_warning ("Unable to create encodebin");
		return FALSE;
	}

	bus = gst_bus_new ();
	gst_element_set_bus (encodebin, bus);
	gst_bus_set_flushing (bus, FALSE);		/* necessary? */

	/* encodebin only checks the muxer and other required elements */
	g_object_set (encodebin, "profile", profile, NULL);
	pad = gst_element_get_static_pad (encodebin, "audio_0");
	if (pad == NULL) {
		rb_debug ("didn't get request pad, profile %s doesn't work", gst_encoding_profile_get_name (profile));
		message = gst_bus_pop (bus);
		while (message != NULL) {
			if (gst_is_missing_plugin_message (message)) {
				messages = g_list_append (messages, message);
			} else {
				gst_message_unref (message);
			}
			message = gst_bus_pop (bus);
		}
	} else {
		rb_debug ("got request pad, profile %s works", gst_encoding_profile_get_name (profile));
		gst_element_release_request_pad (encodebin, pad);
		gst_object_unref (pad);
	}

	/* make sure there's an encoder too */
	enc = rb_gst_encoding_profile_get_encoder (profile);
	if (enc == NULL) {
		GstCaps *caps;
		rb_debug ("couldn't find an encoder, profile %s doesn't work", gst_encoding_profile_get_name (profile));

		caps = rb_gst_encoding_profile_get_encoder_caps (profile);
		messages = g_list_append (messages, gst_missing_encoder_message_new (encodebin, caps));
	} else {
		rb_debug ("encoder found, profile %s works", gst_encoding_profile_get_name (profile));
	}

	if (messages != NULL) {
		GList *m;
		int i;

		if (details != NULL) {
			*details = g_new0(char *, g_list_length (messages)+1);
		}
		if (descriptions != NULL) {
			*descriptions = g_new0(char *, g_list_length (messages)+1);
		}
		i = 0;
		for (m = messages; m != NULL; m = m->next) {
			char *str;
			if (details != NULL) {
				str = gst_missing_plugin_message_get_installer_detail (m->data);
				rb_debug ("missing plugin for profile %s: %s",
					  gst_encoding_profile_get_name (profile),
					  str);
				(*details)[i] = str;
			}
			if (descriptions != NULL) {
				str = gst_missing_plugin_message_get_description (m->data);
				(*descriptions)[i] = str;
			}
			i++;
		}

		ret = TRUE;
		rb_list_destroy_free (messages, (GDestroyNotify)gst_message_unref);
	}

	gst_object_unref (encodebin);
	gst_object_unref (bus);
	return ret;
}


I believe that 

GstElement *enc: must be unref with gst_object_unref()
and
char *str: must be free with g_free()