UndertaleModTool definitely allows you to do all of that easily, although there are two minor asterisks:
1) If you have the original game, UMT will crash when you try to open it, you need a special patched version of the data.win to make it work. I don't remember where it was first shared but I think it was around pages 50-60 of this thread by user jerglekakan. Alternatively, all the mods in the thread are based on the patched version, so you could simply start from a modded data.win from one of the various mods here that will also work. Be aware in general it is hard to extract the changes from a data.win and merge with another one, so choose carefully the one you decide to start with, and try to always keep track of the changes you have rather than just rely on the data.win file. For example, sprites can be kept on disk and batch imported all at once in UMT, so if you keep track of your images in a folder, then that can easily be merged with other mods.
2) Editing sprites/backgrounds is definitely straightforward, but audio might be more complicated, it is possible nevertheless, you can ping me if you get crashes.