Converting an old (pre-DCC era) engine to sound is, in my experience, much harder than just converting it to DCC. You have to get the decoder installed and find room for a speaker and it's enclosure. In some cases, I've just given up and bought a new version of the engine and swapped body shells...although in some cases, the new bodies of the diesels are much better detailed than the old ones.
Anyway, non-sound DCC decoders aren't that pricey compared to sound decoders, for the cost of one sound decoder you could buy like 3 non-sound ones to convert your old DC engines with. Maybe look into ones with some version of "keep alive" or "power pack" if the old engines - like many old steam engines - don't pick up power all that well.
Anyway, non-sound DCC decoders aren't that pricey compared to sound decoders, for the cost of one sound decoder you could buy like 3 non-sound ones to convert your old DC engines with. Maybe look into ones with some version of "keep alive" or "power pack" if the old engines - like many old steam engines - don't pick up power all that well.