In versions before 1.8, you could use the resource type "link to file or website" and point to an mp3 file and, if you had also specified that it display in the same window, it would be filtered and a flash player interface would appear. This no longer seems to occur as of version 1.8. I also tested with a .flv file and similarly, it only offered to download the file, there appeared to be no way to get it to play in place
This may be due to the forms being rewritten as the display in same window options have changed slightly, or it may be to do with the rewrite of the filter code. The rewrite seems to have introduced IE specific code, so it may need to be tested on both IE and in a non-IE browser.
This is not directly linked to this bug, but I would suggest that it might make sense to reuse the flash player currently used for .flv files, to also play .mp3 files in this case. This would share code and be slightly more aesthetically pleasing. The only feature lost would be the "pan" control, which I believe is effectively useless. I had always thought that it acted as a "balance" control that allowed you to listen to only one channel, but it appears it just moves the same audio from left to right.