For my "regular" blog, I just use a WP audio plugin that doesn't require a direct upload into the post (it needs specific tags). To get that into my Tumblr blog as a new audio post, I can always just start a new audio post in Tumblr and then copy and paste the text from WP, etc.
However, for all of my older posts not yet synced with Tumblr, I want to get them into Tumblr as audio posts with the original post date so that I can utilize my Tumblr theme's audio player. I was able to do that with the Tumblrize plugin by uploading the relevant audio file into a previous post (to get the link which then allowed me to select "audio post" in Tumblrize), and then I would "Update" the WP post, resulting in an audio post in Tumblr with the original date.
If you followed that, I'm just hoping that NextScript might be able to allow me to do the same. Including an audio link in a Tumblr text post just doesn't look as nice (http://reselect.tumblr.com/post/31349261845/the-corin-tucker-band-doubt) as one that comes through as an audio post (http://reselect.tumblr.com/post/30507560415/to-say-that-chuck-berry-invented-rock-n-roll-is), if you see what I mean.