While I can't tell you whether the speaker in the Roman Colosseum could be heard I can show some parallels with Greco-Roman theater.

Greek odeums and Roman amphitheatres (Note the similarity with the interior of the Roman Colosseum) had incredible acoustics. If you sat in the back row you could definitely hear what was going on. I've been to one in Greece and I can personally attest that this is the case. I've heard certain people say that certain places in the seats could allow you to hear whispers on stage (Not sure if this is true though).

Vitruvius's De architectura also shows us that the Romans definitely knew about proper acoustics and could achieve the feats of carying volume which I described above.

Below is an excerpt from Book V Chapter 3

6 The voice arises from flowing breath, sensible to the hearing through its percussion on the air. It is propelled by an infinite number of circles similar to those generated in standing water when a stone is cast therein, which, increasing as they recede from the centre, extend to a great distance, if the narrowness of the place or some obstruction do not prevent their spreading to the extremity; for when impeded by obstructions, the first recoil affects all that follow.

7 In the same manner the voice spreads in a circular direction. But, whereas the circles in water only spread horizontally, the voice, on the contrary, extends vertically as well as horizontally. Wherefore, as is the case with the motion of water, so with the voice, if no obstacle disturb the first undulation, not only the second and following one, but all of them will, without reverberation, reach the ears of those at bottom and those at top.

8 On this account the antient (Old spelling for ancient in the translation) architects, following nature as their guide, and reflecting on the properties of the voice, regulated the true ascent of steps in a theatre, and contrived, by musical proportions and mathematical rules, whatever its effect might be on the stage (scena), to make it fall on the ears of the audience in a clear and agreeable manner. Since in brazen or horn wind instruments, by a regulation of the genus, their tones are rendered as clear as those of stringed instruments, so by the application of the laws of harmony, the antients discovered a method of increasing the power of the voice in a theatre.

So it seems evident that the Romans had a good grasp on acoustics and were definitely capable of building constructs that could carry the voice over a far number of rows. While I still can't say about the Roman Colosseum specifically, it would be entirely possible that it was constructed in this manner and could carry voices far considering how similarly it was built to Greco-Roman theatres.