If the melodies are yours, then the accompaniment you create with Songsmith is also yours. In other words, Microsoft doesn't keep any copyright or ownership over your Songsmith creations.
Of course, if you record someone else's melody into Songsmith, ownership of the results is more complicated, and we can't help you there (i.e., Songsmith won't help you circumvent copyrights on melodies).
What if I create an original accompaniment using Songsmith, but without a melody? In other words I turn off my vocal. If I am using my own chord progression ideas and mix of instruments, is this still "my song" and am I able to use it for commercial purposes?