No sound when exporting project as HD video (Missing AAC codec)

Hi there everyone!

I am facing a problem when trying to export a finished project from iMovie as an HD video, it has no sound!

Here's the scenario:

MacBook Pro (Mid 2010), OS X El Capitan 10.11.2, around 90GB free space on the hard-drive, iMovie '09 (Version8.0.6)

The project is finished in iMovie, ready for publishing in HD, when I preview it there, no issues with the sound. However, as soon as I try to export it and select HD as the output quality, the .mov file will open without playing any sound in Quicktime. If I select the lower quality "large" it gives me an .m4v file with sound. Even when I select "export with quicktime" and manually select the audio options there (256 Kbps AAC) the final version will not have any sound.

A few months ago I worked on another project, also exported it in HD. When I check the file info of the earlier exported project vis-à-vis that of the current one, it will say "Codec: H.264, AAC" with dimensions "1280x720", when I export the current one it is missing the AAC whilst having the same dimensions.

I am really no expert on such questions, but I have tried to look for option in iMovie, no luck. Maybe somebody has encountered a similar problem or knows why this does not work. I would greatly appreciate any help.

Thanks in advance!!