Some early sound decoders from ESU Loksound were installed on some Marklin models in the late 90's and early 2000's. The only sound function was a horn, or whistle. Some have pin connectors between the motor decoder, some don't. If your's has the 21 pin connector, you may be able to exchange it with a new mfx smd decoder. if you did one before, without pin connections, then you know how to resolder the wiring.
The new smds decoders come with or without sound files already loaded for steam, diesel, or an electric. Or the dealer can custom load a sound file. Not sure if you can still do it the old way, or you need a decoder programmer tool.
I have the BR 120 with the old Loksound lame dull horn. I'm not willing to tackle an upgrade. I have newer Loks with great sounds.

Keep up with the play Jean, that combination will only deal with msd/3 and mdt/3 decoders. The problem is he has an older decoder, and I don't believe there are sound files available for them, let alone a programmer.