^
Hmm. I looked in FileAssociations.xml myself just now. As you can see below, it already has both the x- form and the non x- form mime-type in its database. So my idea will not solve your problem.
<MimeAssociations>
<Audio>
..
<Item Name="aif">audio/aif</Item>
..
<Item Name="aiff">audio/aiff;audio/x-aiff</Item>
PS In case you are interested. The reason why I had originally made this suggestion was, (based on a similar experience with another file type), that there seem to be some renderers which can download music tracks where the ContentType = audio/x-something (the x- form), but which fail to download if the ContentType = audio/something (the regular form). And furthermore, there seem to be some players where the x-form works on push and not on pull, and vice-versa. So I suspected that perhaps your renderer is one of those that behaves this way for audio/aiff resp. audio/x-aiff ..