I think you'll need to be a little more clear about what you are trying to do. What does it mean to "join" an audio file with a video file?
Mux them into a .mov container file. Put video track and audio track together. Video track is H.264, audio track is AAC. That is standard for .mov HD video.
iMovie will re-encode the audio and video, which is entirely unnecessary.Forgive my ignorance. How is that different than adding the AAC track as the soundtrack to the H.264 file in iMovie and exporting to .mov?
+1 for Subler.I think subler works on PPC macs.