i modified your serial event code until it worked here, so i will not
again work with yours.
first, the minim error ?
i expect that files are under data/…
and processing can read from sketchpath and datapath,
but i recommend to use absolute path,
you could however change to
String song = "groove.mp3"; //______________________ select song
other thing is that i recommended with my code example to
separate the song logic from the serial logic,
in my example then also can start from keyboard,
using the same loop/pause logic
why you not repaired the TOP POST CODE posting?