Something very similar happens in DC mode.
Reversing the wires from the power pack fixes it. Apparantly MRC is enforcing the rule of never backing up at higher speeds than idle.
I have just purchased a Digitrax Zephyr and I want to use my MRC Sychro Sound Box with DCC. According to instructions, The red and brown wires are attached to the DCC Track Out (I assume this means attaching them to the track) The yellow and white wires are not used in DCC mode and the other two wires go the speaker. I have set up the Sound Box in this way, put a locomotive on the track and programed the address, etc. The locomotive is correctly programed and when I run it the Sound Box will only give the idling sound (Diesel) and the function keys on the Zephyr will not operate the Sound Box auxillery sounds. The instructions for the Zephyr offers no help, at least to me. I don't believe there is a problem with either unit; I believe I just don't know what I'm doing. Any help would be greatly appreciated.
Ray