I don't know what type of metal it is, but if you have silver-colored input jacks like 99% of boomboxes do, should you always use silver-colored plugs? Or is it OK to use gold-plated plugs? Does it even matter?

it does not matter at all ,some prefer gold only because its a better conductor ,no other reason.

So no problem mating gold with nickel/tin?

none that i'm aware of ,never had a problem using either type.

The reason for gold plating
is that gold will not corrode
giving a better more reliable connection.

That mating problem is for aluminum wires
which is rarely used.

Thanks. And boombox inputs are not made of aluminum? What are they, tin? nickel?

Not aluminum,
aluminum does not solder.

Not sure, tin or nickel plated maybe.