I have two of those engines and the only thing I have found wrong with the sound is a very weak horn. I haven't tried to change it because it would be a waste of money to purchase the required Digitrax programmer just for this one train set, especially since T55 has gone out of business and I will probably never purchase another Digitrax SoundFX decoder or decoder equipped locomotive.
If I were going to try changing the decoder, I would choose a QSI Revolution A. Much better sound quality than the Digitrax SoundFX. Changing to a better quality speaker may help, too; unfortunately, Digitrax decoders use an odd-ball 32 Ohm speaker instead of the industry standard of 8 Ohms.
Did anybody changed the sound of a Tower 55 ES44AC? I don´t like it (has Digitrax DCC & sound factory installed), and I don´t like it. Thanks