By the other hand, when somebody uploads videos on youtube he/she has 2 options about the licence: standard youtube licence and Creative Commons Attribution license (reuse allowed). The default license set when uploading any video is the standard youtube licence, so when there is a different situation the owner of the video must select the CCA license.
The standard license can be found at Youtube terms and services and it says (among other things):
Content is provided to you AS IS. You may access Content for your information and personal use solely as intended through the provided functionality of the Service and as permitted under these Terms of Service. You shall not download any Content unless you see a “download” or similar link displayed by YouTube on the Service for that Content. You shall not copy, reproduce, distribute, transmit, broadcast, display, sell, license, or otherwise exploit any Content for any other purposes without the prior written consent of YouTube or the respective licensors of the Content. YouTube and its licensors reserve all rights not expressly granted in and to the Service and the Content.
The Creative Common Atribution license says, among other things:
Creative Commons licenses provide a standard way for content creators to grant someone else permission to use their work. YouTube allows users to mark their videos with a Creative Commons CC BY license [attribution - reuse allowed]. These videos are then accessible to YouTube users for use in their own videos via the YouTube Video Editor. Attribution is automatic under the CC BY license, meaning that any video you create using Creative Commons content will automatically show the source videos' titles underneath the video player. You retain your copyright and other users get to reuse your work subject to the terms of the license.
... but the most important thing is the next one: By marking your original video with a Creative Commons license, you are granting the entire YouTube community the right to reuse and edit that video.
Of course, almost nobody is interested in converting to mp3 some home made video and they are searching for the music they love to listen, but that's another story. From youtube's point of view, the legal problems are covered by their licenses. Everything else have to be resolved by the 3rd parties that own the copyright when the rights are being infringed by downloading the video.
Once the base (how the youtube video is obtained legally) is covered, the mp3 conversion must be taken into consideration. The most important thing is the software involved: there are lots of free software that do the job, but you have to pay for some of them. Using the free tools is no problem (at least legally), but using the paid software with cracks and licence codes provided by key generators (exactly: without paying) is not legal. Don't count on the anonymity when you are connected to the internet, the software developers know about the methods to avoid paying for license and they are creating new methods to catch you.
