Yeah, alright, BUT...
if you remember, the C64 datasette (C2N) uses to pause the tape when the commo has found a program. The computer displays the title of the program and waits for the user to press a key. Usually that's the space key, which starts the loading process (the tape un-pauses). You could also press the Run/Stop-key to abort loading if that's not the program you are seeking.
So, when using the method described above with the MP3 and the adaptor, I guess the MP3 is running along instead of pausing and not "waiting for the computer". That way, the C64 misses the first few "seconds" of data.
Only way I can think of to deal with that is to restart the MP3 before confirming the loading procedure by pressing the space bar. That will bring some "useless header data" to the C64 which is anticipating the program code to start right now.
Does the C64 skip the useless header data in the second run? Or do you pause the MP3? Manually?