Hmm... never noticed that before, but in any case, it's not the speaker--that's to the right of the PCI-zone fan.
After breaking out a flashlight I'm guessing that it's a blower either forcing air down behind the motherboard to keep it cool, or sucking air up from behind it and venting out the back past the drives. It's shaped like a blower, and there's power going to it, and it has what appears to be an inlet or outlet on the back end (behind the hard drives near the top). Plus, there should be 9 fans in my DP, and I can only find 8 (2 on each end of the processors, 2 in the PSU, 1 PCI zone, 1 drive zone) if I don't count it, so unless there's one I'm missing (somewhere in the PSU?) that's what it's got to be.
I can't get past my drives to check if there's any airflow and if so which way it's going, but the fact that there's no dust on the "vent" I can see indicates that it's probably sucking up from below and blowing out the back, if that's what it does at all. That'd make more sense in terms of airflow anyway, as it'd match the direction of the drive zone fan. Either that, or it never runs.
