The MS that I just finished was a lot darker than what I generally write. We had a dead wife and all kinds of baggage between the H and h, and while there was humor (I simply can't help but add some!) the overall tone was a little bit heavy.
In contrast, the MS I just started is much lighter. Of course, a lot of that is owed to the situation and the characters. They're not coming from as dark of a place, which makes the overall feel of it much different. And so much depends on who the characters are.
My heroine, Caitlin, has just discovered her fiancee in a very compromising position with her sister, and while it's a heartbreak for her, she's handling it with a little bit of humor. Not everyone, or every character, would handle it this way, but after the twelve hour crying jag she's more interested in moving on than wallowing.
As this is my thirteenth MS *screams* I've noticed that, while my voice is my voice (and re-reading my own writing I can get very tired of hearing myself 'talk') the tone of it does change, even though my MSs are all aimed at the same line. So much depends on those characters!
In my third MS I had a heroine who suffered from foot-in-mouth disease and tended to just spout whatever came to her mind without censoring it. That, of course, made the tone of the MS lighter and added a bit more humor naturally. I've also had characters coming from a place it's hard to find humor in, and with that the overall tone was much darker.
I really enjoy writing both, just as I enjoy reading both. I like a good laugh and I like a good, heartfelt cry. If I can get both in the same book, as a reader or writer, all the better!
So, how 'bout you? Do you find the tone of your voice changes? Or are you a little less schizo than me? :-)