I am not sure what that is to the right of the subwoofer, but I did find some additional details on the speaker setup. It looks like all mazda3 trims have the 3-way speaker setup at the front, along with midranges in the upper rear doors. The Bose setup adds a center speaker, subwoofer and a pair of additional rear satellite speakers on the rear deck.That's definitely a large speaker with Bose sticker, surrounded by 3 small speakers, but why 3... they are not symmetric.. weird hmm..
Speakers are not boxed, that seems amateur... volume is not fixed as it can change depending on trunk load...
Mazda conducted a series of new studies into how sounds are transmitted through a car’s cabin. This led to more efficiently improving sound quality by placing the bass speakers in the corners where the sound is amplified. To avoid the tonal quality of mid- to high-frequencysounds from being affected by reflected sounds, the other speakers were positioned on the left and right sail garnish and upper sections of the door trim where the sound is transmitted directly toward the occupant’s ears. This creates a more natural sensation to distinguishing the sound’s point of origin and enhances the depth and clarity of that sound.
The 8-speaker standard audio system uses a 3-way layout, with 3L woofers positioned on the sides of the front cowl, a pair of 2.5cm squawkers on the sail garnish and 8cm squawkers on the upper section of the front and rear door trim. The 12-speaker Bose®sound system adds an 8cm center speaker, a pair of rear satellite speakers, and a subwoofer. It also features custom tuning to provide more powerful bass and higher audio quality.
Even this entry-level system offers a number of advanced features, including the ability to focus the sound on the driver if (s)he is alone, or to spread it around to other riders.