Just a thought here:
If you create a piece of software in Blitz that allows custom playlists, even if you filter all files with the .mp3 extension, mp3's can still be played (by simply renaming them).
Therefore you would either have to check the file for an mp3 header and filter that out or pay for the license since the decoder is still functional (even though you have not distributed any sounds encoded in mp3 format with the software).
If you create a piece of software in Blitz that allows custom playlists, even if you filter all files with the .mp3 extension, mp3's can still be played (by simply renaming them).
Therefore you would either have to check the file for an mp3 header and filter that out or pay for the license since the decoder is still functional (even though you have not distributed any sounds encoded in mp3 format with the software).