are there multiple spaces in the filename? if so, the following happens (~ equals a space since multispaces are stripped here too):
hello~~there.mp3 (tries to get played by mirc)
hello~there.mp3 (is what mirc tries to play cause of multispace-stripping but the file does NOT exist cause it is really a multispaced file)
I had that same issue till I found the dual spaces in a file and removed the excess spaces
