Music I'm Writing: Fantasy for Viola and Piano in B flat Minor

Spent the morning writing a short piece for viola and piano. I've called it a "fantasy" for want of a better term. I think it's in B flat minor, but keys remain something of a mystery to me, as do modes. This started as an exercise I set myself to write something in the odd Locrian mode, which I like the sound of, but I don't think I succeeded. Still, I like the result. The instruments are synthesized by the computer. The piano sounds reasonably good in the original file. The viola sounds rather artificial. The computer version is good enough to get the idea, but the compression of the MP4 format further distorts the sound, so don't expect concert hall quality. For more about my efforts at composition, see earlier posts on the subject. Click the link below to download a 2.8Mb MP4 file. 

Fantasy for Viola and Piano, in B flat minor