Audio/video out of sync after export

Question

My son Charley and I are working on a simple project, where we have imported an MP4 file into Premiere Pro, then edited it, with no problems, but when he exports the file (either to an MP4 or FLV), there is a small but noticeable audio-video sync problem on the exported file. There does not seem to be a sync or other problem with the source file, nor with the edited file as viewed within PP. But on the exported file, the audio seems to run slightly ahead of the video.
When we upload one of these exported files to Youtube or Vimeo, the sync problem seems to become more noticeable than in viewing the exported flv or mp4.
We cannot find a solution in searching the Forums. Charley consulted with another more experienced videographer, and they tried for hours to no avail.
Charley is using Premiere Pro CS5.5 on a Mac (MBP Mid-2009 2.53 Ghz Intel Core 2 Duo, using OSX 10.6.8).
We see lots of discussion about frame rate. Charley put the settings to 'match source' but maybe there is something else we're supposed to do.
He can post screen shots of all the settings, if that will help. Adobe provides no phone support for PP and just sends us back to this Forum.
Any help  much appreciated.
Michael Lanning

Answer 1

Kevin: OK, that's helpful. Charley is at work until 3:15 Eastern; in the meanwhile, I'm trying below to reply to your inputs; sorry if some of my questions are illogical, just reflecting my confusion.
You say to match source settings, including audio sample rate, and (I think) that we should do that by creating a new sequence at which time we select the clip and drag it to New Item button. Perhaps doing that would make some of the following questions irrelevant, I'm not sure, but here they are:
We did not check the box "Match Sequence Settings" (just under 'Export Settings'); should we have done so?
On Presets, and using FLV vs H.264:
Do we need to leave the 'Match Sequence Settings' box Unchecked if we want to use a YT or Vimeo Preset?
FLV does not seem to have a pre-set for YT or Vimeo, but does have one for 'Web.' H.264 does have YT and Vimeo pre-sets. As I've mentioned, we have been trying to make this work with FLV, as Charley found that H.264 was giving us (worse) sync problems. Maybe we should go back to H.264.
You say that "if these things don't work, "...try an similar H.264 preset to upload to" YT. Do you mean to suggest that we try exporting in H.264 instead of FLV? (If that's not what you mean, and if we still work with FLV, then I don't know what "an similar H.264 preset" would be; do you mean like the 'Web' preset?
Whether FLV or H.264, I'm unclear how ensure audio sample rate matches source.
Codecs listed on source file are H.264, AAC; does that imply that under 'Basic Audio Settings' on Audio settings tab, we should use an AAC codec rather than the MP3 now indicated?
Should we be determining the bitrate settings of the source (ie, right click the file within PP to find the properties, including bitrate?) and then using that bitrate?
Or do we resolve all this about audio sample rate simply by creating a new sequence as discussed above?
I hope these questions make sense. Thanks again for your help.