I think your input and labels should be something like this:
x_train shape (batch, 128, 800, 2)
y_train shape (batch, 7)
y_train is calculated from your final.txt file using timestamps.
Lets say your first spectrogram starts from 0, ends at 72.13. The label should looke like this [n, s1, i1, s1, i1, s1, i1]. Then you change it to class id: [1,2,3,2,3,2,3].