How can I convert .rofl replay files to .mp3 format?
As far as we know, converting League of Legends Replay Files directly to standard MP3 Audio File format (.rofl to .mp3 conversion) not possible. Game replays usually contain game engine instructions only, not recorded audio data.
A quite complicated option would be to load your ROFL file in the game itself, and record the replay with a screen capture utility, such as Fraps or Camtasia. Then simply export the resulting recording MP3 format.
A screen capture and video editing software
A screen recording utility