Hi morroklump, welcome to Fusion and nice first model. Dan's receommendation is completely correct, based on the way you created this model. I'll break down what you did.
the main volume of the box you created using a primitive.
this gives you a very fast way to build form and explore ideas, however, it's not the best method if you want to control your design.
the method you went trough to create the speaker shape is really where you should start if you know you want to control your design.
steps worth trying
start with a sketch of the main shape, you can dimension this and add the main features in this sketch as well, speaker holes/location, ports. connector locations, etc.
best practice: keep your sketches as simple as possible, stay away from adding fillets or secondary level of detail that will need to be added to your model
Create the main extrusion
now you can go through and add the remaining features using new sketch + project (from your original sketch)
good luck and we're happy to answer other questions to help get up and running
I've added this into the model you posted. have a look at what I started and you'll be on your way, because you've got the generaly prinicples, just need to put them together a little differently
hope this helps,
Jamie Gilchrist
Senior Principal Experience Designer