Make YouTube remember the volume level you prefer
Normally, YouTube remembers the volume preference of its users. For example, if you find most of YouTube clips too loud and you prefer an initial level at half the volume or even less, you would just decrease the volume once, to have all subsequent clips follow your setting.
If YouTube stopped remembering your volume preference, you or someone else, or a fresh flash setup on your machine, changed the global settings of your flash player. Here is what you can do, to give back to YouTube its memory.
Right click on a clip and select the Global Settings option. In the first (default) card that appears, make sure that you check the option Allow sites to save information on this computer.
If this option is not checked, YouTube can not save your preferences, which means that each clip will always start in full volume. Enjoy!