You shouldn’t need to change the speed of either. Most likely your two files are different lengths (ie: movie is 91.3 minutes, audio is 89.8 minutes). It will be a matter of trial and error to get them into sync, especially since they are likely from different sources.
Basically, what you need to do is get the files the same size. If the audio is out of sync the same amount of time throughout the movie (best scenario), it is simply a matter of adding or removing audio from the beginning of audio file to get them into sync. If the audio sync increases throughout the movie (worst case, in which I would delete and find another source), then there is much more work to be done. In this case, the audio file is most likely shorter than the video. In which case you would then change the length of the audio file (start with the length of the movie file) and then test. You will want to keep adjusting the size until you can get the audio sync difference continous throughout the length of the movie. Once you have a continous sync difference, then you add or remove audio from the beginning of the audio file to get them into sync. I use GoldWave for audio work, but there are likely other free alternatives.