Solved by ptrblck in post #2 
              
              
                In the default setup nn.RNN (as well as e.g. nn.LSTM) expect an input in the shape [seq_len, batch_size, features]. 
I assume you are working with 128 frequencies and a variable sequence length in the last dimension. 
If that’s the case, you could remove dim1, which seems to be an unnecessary channe…