Synthetic Audio Library. (SEAL) I don’t know the URL for it, but if you want I can send you the ZIP, which has source/includes/docs and also send you a pre-built library and .dll (it’s a pain to build ) It is just too easy to use. If you aren’t looking for all kinds of features, you’ll love it.
or try Midas. It’s supposed to very good, but i never used it.