For a tree plant a Rowan - protects agaisnt wichcraft - and also provides berries for for a variety of birds in winter. For a hedge I would get rid of the conifers and plant beech within a few years time some honeysuckle, travellers joy, etc to work their way through it and again feed the birds. I am assuming that the hedge you have is some form of Lawson Cyprus which is a greedy plant and is almost impossible to underplant with anything more interesting. However, next to a busy road, it will screen noise.