Is it possible to use the midi format to modify an mp3 file? I want to change the time signature, and copy/paste additional notes. Let me know. Thanks
Without recompression - you can do basic editing on mp3 files (gain/normalize, fade in/out, cut/copy/paste, trim) - with MP3DirectCut for example (free), but everything else requires re-encoding, at least on re-edited segments in the way you wish to.
Here is what you can do. Load your MP3 file into some DAW (like Reaper). Load your midi file and associate desired virtual instrument (vsti), synchronize both, and export. If you don't want to get "bad artifacts" (compression related) - when using mp3 - give yourself 256 or 320kbps bitrate.
Midi and audio streams are two different directions in audio production.