I did in fact swap out the Audio icon for a custom-made Review icon, but this solution is not ideal. The Audio tumblog forces you to add an audio link; if you leave the link blank, you still get an audio slider.
This can be changed from woo_posts.php file located in (library\tfuse_mods\thememodules) directory, from there you can change the layout for each post type.
If I cannot get that (Category icon change) to work, perhaps I can ask you to create a "Test" post with any icon (other than "Blog") so that I can see how it was done. Would that be possible?
It seems that you didn't understood how this works. For posts are used tumblogs, for ALL posts, it doesn't matter that the post is a part of a category or is on a specific category post. You can choose there only one tumblog for a post. There the only way to change the icons to other than default would be to replace default image with the new one. What I told about category icons, those are shown ONLY on portfolio and gallery pages as you can see here or here. Hope now all would be clear.