Ubuntu's commitment to only include completely free software by default means that proprietary media formats are not configured 'out of the box'. See Ubuntu's Free Software Philosophy and the Free Formats page for a more comprehensive discussion of these issues.
Notable part from the 1st link:
Ubuntu 'main' component licence policy
All application software included in the Ubuntu main component:
- Must include source code. The main component has a strict and non-negotiable requirement that application software included in it must come with full source code.
- Must allow modification and distribution of modified copies under the same licence. Just having the source code does not convey the same freedom as having the right to change it. Without the ability to modify software, the Ubuntu community cannot support software, fix bugs, translate it, or improve it.
and from the 2nd link:
What is a Free format?
Some file formats are proprietary, which means that they are owned by a company or other organisation. Sometimes, the owners of such formats charge licensing fees or impose legal restrictions on the use of their formats. This means that people may be unable to use or distribute these formats without first paying a fee or applying for a license.
A Free or open format is one which can be used by anyone, free of legal restrictions on how they use the format. Free formats are very popular - the World Wide Web is based on the open HTML standard. Ubuntu supports many free formats and the open-source community as a whole encourages their wider use.
Restricted Format MP3, AAC, WMA, WMV, MPEG-4, H.263, doc, xls, ppt
Preferred format Ogg Vorbis, Ogg Theora, WebM, OpenDocument format
edit: are you sure Lubuntu installs
lubuntu-restricted-extras by default? A small search would suggest not. This suggest Lubuntu uses the same method as Ubuntu:
When you installed Lubuntu, you had a chance to check a box enabling you to install restricted addons that enabled things like MP3/DVD/Flash playback. If you forgot to do this, you can still install these packages by opening the Lubuntu Software Center (version 12.04 and later) in the menu under System Tools. Search for “lubuntu-restricted-extras” and, when the selection loads, click “Install.”