On some of the backups I have done the compression has been quite high
Hellboy was at 50% for example,
even after taking foreign languages out it was about 58%, my question is, I have often seen the exact same audio repeated in the main title.
For example AC3 5.1 English 328 MB has come up twice, whats the difference? can you get rid of one and be confident of it playing fine?
I've never seen that before, usually there is some difference, ie 2 channel vs 6 channel. I'm not saying that there is not though.
If you are not sure which audio to use, try previewing it. Start the movie in Shrink's preview window. Right click within the preview window and a menu will open. Select Audio. Now you can listen to each individual audio stream and figure out the one you want. Once you have determined which one, check the corresponding box and uncheck the rest.
