i have a peughot 307 which had head gasket problems because of water pum?

i have a peughot 307 which had head gasket problems because of water pump leaking.this has now been fixed.the head gasket has been skimmed,cambelt changed,water pump changed as well.antipollution fault has appeared on my multi dispay system,the mechanic fixed as well.but the sound of the exhause system is different, abit louder,how do i fix it pliz.thanks.