You're misunderstanding the word "stretching" here. When you speed up a sample, it's gonna sound higher in pitch, because the waves are closer to eachother, thus raising the frequency of the waves. When we say stretching, we mean speeding up/slow down a sample without affecting the pitch. Obviously, once you've done that, you can slow it down again to the same length and you'll have a different pitch. That's not a very practical way to work (except when you want to do basic intervals like octaves) but it's the same algorithm. We call it "transposing", or sometimes pitch shifting. I'm not sure how to do it in audacity (reason cannot do it, except if you have reason 6 or record).
PS: if you want to shift it an octave down, timestrech the sample to the half of its original length, and then lengthen them again to their full length without timestretching. But i'm sure audacity must have a pitch shifting engine too.