David - Your LGB locomotive was produced by the former LGB company in 2003 - 2005 timeframe and has an Onboard decoder sound system..........the speaker is in the tender connected by the cable from the Mogul. The electronics were made by Massoth. Some DCC system brands don't work well with programming Massoth type DCC decoders.....they don't produce enough voltage. Since you're DCC system has failed, you should ensure whichever DCC replacement system has enough power to properly operate and program all DCC decoders. Some hobbyists are using DCC systems designed for small scales that only produce around 14 volts DCC to the track..........LGB locomotives need 18 - 22 volts DCC.
I'm a ESU, LGB, Marklin, Soundtraxx and Massoth seller, LGB locomotive Authorized Service Center, and also install any of the DCC decoders I sell into customers' LGB locomotives. For example, super wireless DCC system you should consider is the ESU CabControl Digital System which I sell for $400.
If you were getting the locomotive to operate in DCC but had no sound, you should confirm that the cable and plug from the Mogul to the tender is not defective, or that the tender's socket is not defective. This cable transmits the sound to the speaker in the tender, the power to the tender's rear light, and also carries track power from the tender's power pickup wheels to the Mogul. Identify which of the two pins on the Mogul's s-pin cable connector transmit the sound to the tender and then with the Mogul powered up on a test track with the on/off switch in the lights/power on position but no motor, connect another speaker to those speaker pins to see if you're getting any sound or not. At least that can help you isolate whether it's a defective Mogul - Tender Cable/Plug/Socket issue, or whether it's the Onboard decoder not producing the sounds.
If the Onboard decoder is defective, you'll need to replace it with a decoder such as Massoth XLS DCC sound decoder, either the version with the screw terminals or the Onboard version with the sockets, depending on your Onboard decoder's configuration.