Does the browser upload Captivate audio files more than once?

No, I don't have any two clips playing at the same time.  I have them spread out across the slide so that only one person is talking at once.  I'll try to add more space between the appearance of each object.

Do you have overlapping audio? One of the reasons I will choose for audio objects is that they are the only possibility if you want two clips playing at the same time.

If the audio on those objects is playing at all, that indicates the browser HAS indeed downloaded them. Have you experimented with moving the objects further apart to see if the audio is then completed properly before the next one starts?

Your issue with the audio getting cut off before the end may be due to the fact that some mobile devices will only allow ONE audio clip to play at a time, and the preceding clip will be prematurely terminated if another clip is loaded to play.

HTML5 content doesn't always preload required component files well before they are needed for playback (like they are with SWF content).  The objects required for each slide might only be downloaded when you get to that particular slide. 

So if the audio file is requested from the server and takes a few seconds to load (depending on bandwidth) you can find that it will get cut off by the start of the next audio object.  It's going to be impossible to predict what the bandwidth (and hence the download time) will be for each individual user.  You may need to simply allow more time between the objects involved in these 'conversations' to allow for the delay in download.