I don't think you can do this easily. In addition, you shouldn't do this. The machine belongs to the user and, if they'd prefer to stream your MP3s rather than downloading them, it's their decision.
My own solution would be to provide a link for people to complain that they're streamed rather than downloaded (well, complain about anything, really) and let the community (including yourself) provide solutions.
Then, if someone states that it always seems to play rather than download, you can work with them to figure out they're using Firefox and QuickTime and tell them they can either right-click-save-as or reconfigure Firefox.
I will say that there will be ways in which you can achieve this, such as changing the MIME type or file name that's sent down, but my comment that this should be under the control of the user stands. And anyway, a determined user can quite easily re-configure their browser to stream whatever MIME type you decide to use so you still can't guarantee it.