Listen...A ditch witch will destroy the grass, and make a mess that will take hours to clean up.
A ditch witch is designed for electrical use where you want to be 16 or more inches deep. Not to mention a ditch witch will chew up the irrigation piping, and take hours to repair.
I do alot of exterior systems very similar to what you described. I hire a irrigation contractor to run the conduits. They use a vermeer machine that slices the ground and at the same time, buries a 3/4 inch irrigation conduit. It does minimal damage to the landscape, and the irrigation contractor has ALOT of experience in blowing lines under walkways, and he will steer clear of the existing irrigation piping already in the ground. After you plot out your locations, pull direct burial to your speaker locations. I install an electrical underground "access lid" near the house, and all the conduits run to there. From there, I run PVC into the house.( USE 12 GUAGE SPEAKER CABLE )
If the runs are more than 200 feet from the house, try to locate the exterior AMP in the pool house or gazebo and then run balanced line from the system at the house to the amp at the pool house or gazebo. This will make things ALOT easier.
Hiring the Irrigation contractor is not expensive, and your employees will thank you for not making them "trench monkees" for a few days.