I've done precisely that (played the same file I'm uploading) many times without ill effects. Go for it.
It comes down to how a given application chooses to lock the file as it opens it. I forget the actual names of the flags in the Windows API, but there are a lot of choices you can make when opening a file programatically.
An app can choose to lock the file so that only it can read it. For instance, if the app intends to modify the file, it could say "everyone else: hands off".
Or, an app could lock the file so that anyone else can read it, but not write to it. It's like saying "okay, you guys can look at this file, but if you want to change it, forget it because I'm trying to look at it at the same time". I'm pretty sure WinAmp uses this flag when it's playing the file. Since Emplode seems to use the same "everyone can read" flag, then it's OK for Winamp and Emplode to see the file at the same time.
Interestingly, WinAmp will release its locks when the file is stopped. For instance, I can open a file in WinAmp, play it, and press the Stop button. Then I can open the same file in MP3Trim, make changes, save the changes, and then press Play in WinAmp and hear the changes. Very nice.