One of the things that may be confusing for newer Android users is the Silent mode. If you're on the home screen and use the volume buttons to enter silent mode (or use any other possible toggle switch or widget you might have), that may not actually mean "no sounds."
Say you're at a conference listening to a speaker that really doesn't apply to you, but you have to be there anyway, so you're thinking you'll play a game on your brand new Android phone or maybe browse the web. It's set to silent mode, so even if a flash video with audio shows up, this shouldn't be a problem, right?
Au contraire mon frère!
If you look at the settings for silent mode, it says, "All sounds except media and alarms are silenced." Okay, alarms are good to keep on, and you would think "media" means playing a movie or music, so that's okay too. Actually, "media" apparently means any application that has audio output. So if you were to visit a web site that had a Flash animation, you would hear the audio even if it was on silent mode. The same goes for games, and even free wallpaper changing apps that have sounds.
If you want your Android device to actually be on silent mode, you have to remember to also change the media volume since that is a separate (often hidden) control. Usually you can access the media volume control by playing a song or video and then pressing the volume down button.
This "feature" has been around since the beginning, and as you can see from some of these forum posts, many would consider this a significant usability design flaw. There are some apps and widgets suggested in that thread that may help manage silent mode a bit better.
Did you ever have your phone play sounds inappropriately even though you thought it was on silent mode?