Instead of creating triangles and trying to fix them later, keep your topology using quads all the time.
It is a common misconception that the topology of an object comes at the end of the process. The topology of the object should always be worked out from the beginning.
A simplified way to do a speaker:
Start with a cube, scale it on the $Z$ axis, and make a loop cut in the middle.
Position the cut to divide the two main sections of the speaker.
Create some vertical loop cuts And subdivide each of the sections with an equal amount of cuts.
select the edges that will become the round part and press ⇧ Shift+⎇ Alt+S (to sphere) and move the mouse to create a circular shape.
Do the same for the top part.
Select the central faces to extrude.
If you select the loop ring in the front you should be able to bevel the holes.