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You can see the outside on the edge of the top speaker (bottom-right corner). The wood is thin, maybe 2mm, and glue applied likely by a machine, yet the slot length looks like it was intended for tuning frequency response. But why wood, and only for this one side of the one slot in an otherwise plastic housing?
In horn speakers (very useful as they are very effective so a low lever amp can drive them well, popular back in the old times. Megaphones also use this for example) you adjust the length of the horn in any kind of way you can (like clogging it up with some solid stuff) and you put foam to dampen in the end part (where the soundwaves go to die, so now they get absorbed instead of being heard), it might be something like that.
It doesn't seem lije a hirn structure, but it can be any other type of enclosure type, like open or closed for example, the volume inside is closely matched to a frequency, so you can adjust that a bit with some solid stuff.
Or it's just to eat up vibrations. Remove it and listen to how it perform.