if the file is not appearing in the switch window, it means that switch is experiencing some sort of error loading it. Certainly the evidence as you have put it would indicate the underscore is posing a problem, but when I added an underscore to files and tried to load them, I did not experience any problems myself. I even tried this on an mp2 file and it worked.
I opened a command prompt and set it to the Switch folder and then typed:
switch "_test_.mp2"
The file loaded fine and it also converted fine to ADPCM wav.
Are there any other characters in these files of yours that are not letters or numbers ?
edit: this is a longshot, but is the underscore by chance in the file extension and NOT file name ? e.g. "test.mp2_"