I am using 64bit Windows 7 with NVIDIA card.
With hi-res downloads that include AC3 the audio track plays all the music and unsynced voices but voices that need to be synced are completely missing.  Any help?

You can manually force Tixati to recheck your transfers by right clicking the transfer in the transfer tab -> Local Files -> Force Check. If this doesn't fix the problem then the file was broken by whoever encoded the upload.

What player are you using? Really sounds like a player issue, especially if you are using Media Player. It handles AC3 poorly. Try using VLC media player , it is a free download.
The other alternative is to recode to AAC with Handbrake or Freemaker converters again both free downloads. If you convert and things are fine it proves it is a player issue not a file issue.