When I have a good pair of headphones on, music helps to sharpen my concentration; it's easier for me to get lost in what I'm writing. Certain songs can really bolster my mood when it comes to writing action scenes.
I definitely use music to help me get into the mood when roleplaying. I even go to the extend of replaying a song over and over til I finish my post. However, I have to have it very very soft and faint. If it is too loud I get distracted and start dancing or singing... o_o
Using music helps on so many levels. I can't even begin to describe it. Which is probably just as well, considering that not many people enjoy death metal these days.
I must be the odd one out here because I find music distracting for the creative process that goes in my head. It either becomes noise to my muse (making me want to shut it off) or if the music is particularly inspirational, will carry me away from the Roleplay entirely. :S
I have used music before. Sometimes, for a certain scene, if there's a song that relates to it or creates the emotion I'm searching to emote in my post, I'll listen to it over and over again while typing. Otherwise, I don't listen to music. It just has to feel right.
Music has become a part of my roleplay in so many different scenarios. I tend to vary my music depending on the scene I'm roleplaying at that particular time, it seems to help the words flow more freely and fluently.