Linux.com :: Ten tips for new Ubuntu users
1. Getting multimedia to work
The default Ubuntu install contains free software only, which means that it doesn't support some popular multimedia formats straight out of the box. This is inconvenient, but the Ubuntu folks have good reasons for not shipping with support for MP3, DVDs, and so forth -- including that software could cause them some legal headaches, or incur some serious fees.
Fortunately, as a user, you don't need to worry about fees (though some of the packages may not be legal due to patent restrictions or restrictions on circumventing copy protection, depending on where you live). The Ubuntu wiki has a page on restricted formats that explains how to get the packages you need. However, if you run Ubuntu on AMD64 or PowerPC hardware, you'll still be out in the cold for some of the packages, since some multimedia formats depend on proprietary software that's not available for those hardware platforms.
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