Go to the Main Menu File ➤ Movie ➤ Export Audio to Movie...
This command creates a new movie file that, with the audio from your GarageBand Project. Select the Full Quality Preset in the next window.
Regarding the DVD part of your question. This has nothing to do with GarageBand and has to be done with a separate app. Apple had the iDVD app in the past, which they discontinued 😠. It was part of the iLife suite and if you still have it, use it. It is an amazing, easy to use app to create nice DVDs with cool looking menus and all.
Hope that helps
Edgar Rothermich - LogicProGEM.com
(Author of the "Graphically Enhanced Manuals")
http://DingDingMusic.com/Manuals/
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