m3u is a playlist. It is not a music file per se. LW can save its playlists as m3u files which can then be opened by programs such as iTunes & Winamp. Likewise a Winamp m3u playlist can also be played by LW's media player by importing it via the playlist.

As far as burning goes. Your app would need to have access to those files. After all, a playlist is simply a reference to the files. If those files have been moved then it won't work. If it plays in your player that you wish to burn from then it should burn ok.

I don't know cd doctor so I can't comment. Not unless it has the ability to open playlists!