The actual process will depend on your final design, but I mocked up a generic hole patterns - a grid and more circular pattern. A single hole with a multitransform might not be enough information for FreeCAD to give you a pattern, but a few holes and some transforms is a lot more for FreeCAD to work with.

Screenshot: https://imgur.com/a/wlUcvs7

FreeCAD file (v0.19): http://joshwhitman.com/speaker-holes.FCStd

The top grill is two holes, offset by 5mm at 45* and then multitransformed linearly to copy those two holes along the X and Y axis.

The lower grill is two holes in a polar pattern 8x, then another hole further out in a 16x polar pattern, and final hole also in a 16x polar pattern. The last hole could be sketched into the previous sketch, or there could be more holes in the final polar pattern.

Hopefully these are some helpful ideas to get you closer to your intended appearance.