I have seen audacity put a little delay when i export a file, for example:
When i export a AC3 file and i import again the exported file, i can see how audacity positions the file with a 5ms positive delay, comparing with the original.
Some lossy compressed formats have this extra bit of padding at the start, some don’t. MP3, AAC and AC3 have this padding at the start. Ogg and FLAC don’t. Also, lossless formats such as WAV and AIFF don’t have this problem.
I wasn’t aware of that, but MP3 and AAC/M4A also add silence to the beginning of the file. MP3 also adds silence at the end. I’m not sure if AAC adds silence to the end. These are lossy formats so they are imperfect.
There is some information about what happens with MP3 files on [u]this page[/u].
If there is silence (or nothing important) at the beginning of the original file, of course you can trim it out to compensate before exporting to AC3.