Hey Dave,
BSP brushes work great for laying out a level and testing basic collision, and allow level designers to build something before level artists are involved. They do not, however, have full lighting and collision support and your options for manipulating settings for BSP geometry is limited. You’ll most likely need to replace brushes you create with Static Meshes.
UE4 does allow you to create Static Meshes from brushes, however, and that may be easiest solution for you. In Details panel with your brush selected, expand Brush Settings and you’ll see a Create Static Mesh button. Once that is created, you can open new Mesh in Mesh editor and give it collision via Collision drop down. From then on, you’ll have full control of collision options for Mesh.
If you would rather use an overlapping volume as a trigger for sound effects, you can just pull it out a bit so it sits in front of BSP, and that should resolve any overlap issues.
There’s some great information on BSP brushes here:
Hope that helps answer your questions!