Obviously, I don't have a 328, but I do have an arch with speakers. One thing is the arches are not very thick, and therefore are not a real good enclosure for speakers. They will do fine with the highs, but no so much with the bass. Mine are mounted on the sides and do a good job of projecting the sound all around the cockpit. I know some of the 318's have speakers mounted in the top facing down, and I'm sure those are plenty audible as well.
Another thing to consider, if you sit at the aft lounge facing towards the stern, the speakers will be right in your ear, which sometimes is not so good when you don't want it loud and happen to be in that seat. I suspect facing down in the top of the arch would eliminate the speaker inches from grandma's ear.
