Having spent some time looking for easy ways to create playlists I came across this little beauty:
Playlist Creator 3
It's a free download - although I'm going to donate because I think it's awesome - and it is so easy to use. Literally drag and drop.
I started out by copying some wma, mp3 and m4a (iTunes) files onto a SD card to test with. I put Playlist Creator on one side of the screen and Windows Explorer on the other and started knocking playlists out like no tomorrow. I'm not certain which playlists are supported by the Audi system but I am 99% sure m3u's are, you can select this at the bottom of the page. Choose the root directory of your storage device as the save location and give your play list a name. BINGO!!! Simply drag the folders or files you want to include into the Playlist Creator window to add them.
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If you intend to use your iTunes music you will have to move the m4a file type to the active side of the list under >Settings>File Type
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I seem to recall someone on the forum once saying that if you start the playlist name with a symbol e.g. underscore, it will always appear at the top of the list when you are viewing your media - might be worth bearing in mind.