They are all hollow. One of mine must have gotten water in it at some point and froze; it is "puffed" up a bit. I have my tower speakers installed on the front cross bar, they are a little more tucked up in the tower and away from knocking my head on them. Also, when I installed my bimini, I had to drill mounting holes into them.

I have replaced that front vinyl as well, and it can be a pain. I tried to keep the pieces in a curve while I installed the vinyl to help reduce the ripples.