Well, it has a SUSI interface, I suppose you could buy a sound decoder that has a SUSI interface so the speed commanded to the main decoder was reflected by the sound card.
But the list price on the sound only decoders are over $200 in the us, so maybe it is a poor way to do it.
I'd swap the decoder out with a sound one and use the XXL somewhere else or sell it. (I know you asked not to change the decoder, but the hassle of adding a sound only one will far exceed just swapping the wires to a new decoder and hooking 2 wires to a speaker)
Greg