All deltarune pages have the songs embedded in them, and I don’t see why it can’t be like that for Undertale. It would be a lot easier to listen to the songs if there were embeds.
They do have song embeds. Take a look at Ruins.
Not all of them do. For example, Battle Against a True Hero doesn’t have an embed.
We use Bandcamp as a source for our soundtrack embeds. Battle Against a True Hero is not present in the Bandcamp listing, so there is no embed available.
Then why not do what the deltarune wiki does and use the Spotify embed?
If you aren’t logged in to Spotify, you only get a short preview of the track instead of the full thing. Personally, I don’t see the point in doing that.
I just haven’t synchronized the gadgets across wikis. I’ll do that eventually.