You can use your mod manager's trainer to force the music to be something else if you really want to have another stage music than Under a Pledge play. But that it works in other modes, I'd almost say it's a safe bet that it could be one of the remaining four stages, to be released at a later date once they've finished ironing out the details. Like the animals are completely untextured. Trying it myself, I started a fight and some of the elk? caribou? not sure ran by, and they're just solid grey, even close up, so it's not a distance rendering thing. The stage just really is incomplete. Though I really do like the effect they've used for the infinite a lot better than what was in SoulCalibur V. If you looked at the background in that, it was circular and spinning and could easily make someone dizzy if they focused on it long enough. This way is a lot more clean and smoothly animated.
Also in regards to your other video where you showed off playing with the AI in Hilde's story, I'm not sure who mentioned Wilhelm appearing in SoulCalibur IV, but that's not true. Not even as a random Tower of Lost Souls character (like they reused a handful of names from SoulCalibur III's Chronicle of the Sword for example, but the characters aren't using similar looks or styles) or anything like that. They must have just misremembered it. That being said, though, Hilde's other knights than Gerhilde that were mentioned in her Soul Chronicle but not seen, Ortlinde and Rossweisse, they were non-playable but did appear as enemies in some characters' story modes in SoulCalibur IV, though they were serving Astaroth, along with a third, Helmwige. Other knights seen alongside Hilde in SoulCalibur IV other than Gerhilde were Waltraute, Schwertleite, Grimgerde, and Siegrune.