I have a Centurion 5, and I can confirm that it does not have a speaker. The POST beep (barring any weird routing through the soundcard or whatnot) comes from the motherboard itself, no case speaker needed. Some cases, notably those produced by 1st tier OEMs, do have internal speakers; yours does not.
Most cases have a tiny speaker for the POST beeps (Red and Black wire). Some motherboards does not have a speaker but all motherboards have a speaker connector the same place as the power button, HDD LED etc connectors are which is usually the lower right corner of the motherboard. The Speaker connector on the motherboard is two pins separated by two pins like this .... First pin is speaker + then comes the 2 center pins that are not used and then the speaker - pin.
back in the dark ages of the 8088 IBM PC-junior up through 80286, 80386, 80486 and pentium (ones and twos) computer cases had internal speakers and in almost all instance there were no speakers on the mobo. programs were even written so you could play tinny tunes on the little 3 inch case speaker, heck, even the bios (cmos) battery was separate from the mobo.
in recent years mobo makers have been putting speakers (piezo buzzers) on the mobo (probably because not everyone could figure out how to connect the case speaker).