Speakers in metal boxes are prone to resonate somewhere. I would be willing
to bet 5 cents that the reason they are there is to prevent unwanted
resonances.

Regarding the speaker common lead, it is common to avoid a common ground,
which could cause ground loop problems (hum). Usually, the manufacturer
knows best and all you need to do is put it back to the way the engineers
had it in the first place, and it will work as it was supposed to.

73, Colin K7FM