Hey Gamma V, try putting the main map music value as 00 and see if it works. In case it doesn't, I can provide you a fix, since I made a little uberASM code for myself that allows variable custom music in a same submap depending on certain stuff. I'd just need to do a little tweak, but that won't take any long.
If you mean for me to try setting it to 00 in Lunar Magic, I tried that once. It did nothing but cause the previously-playing song to continue. I am using UberASM on the Overworld for HDMA gradients. Apparently, that's not what's bugging up the music-loading on the now-split main map. I set up a new ROM and installed every bit of ASM BUT UberASM, but the same thing happened. I checked my other patches for writes to $1DFB and only found them in the SMB1 Flagpole patch, in which they're kinda necessary to make the level clear fanfare play.
UPDATE: Thanks to Kaizoman, the issue with the Submap Split Patch's music loading has been fixed. Now I can move on and finish this hack, starting with 4-1.
This is pretty much your average forest level.
Expect plenty of Wigglers and hostile plantlife.
There are also Lakitus hanging out in some of the pipes, and they're all too happy to pop out and toss a Spiny at you.
Interesting level and graphic swaps, though I also noticed the cutoff and agree that the music used in 4-2 seems rather unfitting.
And since I didn't comment on 3-3, 3-4, or 4-1 either...3-3 seems fine, but I don't have much to say about it. 3-4 looks to be well put together and well decorated. 4-1 also looks pretty all right, though I'd be careful about having too many foreground-obscuring trees.
I wanted to comment about 4-2 as well. This might be a bit cliche, but I would recommend the music from the beehive levels in Donkey Kong Country 2, I think it a would be very fitting. Although to me the level looks strange having a background that is vastly different from the foreground. Maybe go for a cavern type background? Just my opinion though.
I originally considered drawing a cave-like background, but decided to just reuse the foliage from 4-1's background because this beehive is in a tree. Also, Flight of the Zinger seems like it wouldn't be "Mario-like" enough.
Since World 4 is almost done, I decided to go ahead and put together World 5's map.
Yet another mountain-themed fifth world in a Gamma V hack
-------------------- "Experience, lad. You should learn to appreciate it."
-Kup, Transformers: The Movie 1986
I kinda like the flat style you're using here, it works pretty well with the more linear style the hack seems to follow. I think some of the decoration doesn't look too good, though (specifically, the rocks and the climbing stones under both the rock and the castle).
Not much to comment on 4-3, it's a solid level overall. Only nitpick I have is how the camera doesn't follow the player, considering how high they send Mario. It would be great if you made it work when you land on the higher mushrooms at the very least (the one around 0:26 is a nice example). Also, you probably already noticed it, but there's a poorly placed bouncing Parakoopa around 1:10 that dies pretty much instantly, you should move it back a bit.
4-3 seems decent enough, and I quite like what you did with the design and aesthetics of the World 4 castle. The World 9 map looks pretty cool as well. And yes, 5-1 also looks good. (Seriously, what is it with you and World 5 being mountain-themed? I've also noticed a distinct theme of W3 usually being a desert, W4 usually being a forest [occasionally the other way around], one of W6 or W7 being ice, and one of W1 or W2 being a beach.)