Thanks for playing!
The abyss area is very much meant to have no music, but I did intend on adding some kind of ambient sound to fill the silence that I ended up never getting around to before the deadline hit. As it stands it does feel kind of awkwardly silent there, yeah.
There's def still a lot of jank to work out in the exact behavior of the slash, yeah - making this has made me really appreciate some of the finer details of how it works in Specter of Torment. The direction picking is def still wonky - it doesn't take into account which direction you're actually facing when picking a target and the up/down distinction feels like it could maybe use a bit of tweaking as well. I'm a bit disappointed you found the i-frames too unreliable to actually use it for combat, I found slashing back and forth through enemies to be really satisfying (which does make me hesitant to go with Specter Knight's bounce as a solution to some of the jank, even though I'm definitely coming to realize that it does solve a lot of the problems I've been having). I def need to do a little more to prevent the player from being hit during a slash, I think it's mostly just that the i-frames are not really generous enough - I mentioned in another reply that they don't actually start immediately but they also cut off immediately when the slash's acceleration does which does sometimes leave you inside the enemy hitbox when they cut off, causing that "phasing in" phenomenon you're describing.
The extra height from the wall jump is something I noticed occasionally but could never quite place exactly what was triggering it - your description of it does sound correct though and I think I have an idea of what's causing it now. I'll def want to try to fix that as it does feel very weird and inconsistent with how the wall jump normally behaves. As for the hover thing I did actually properly notice that when testing and intentionally left it in since I thought it was a neat strat, but I didn't really test it enough to get a good gauge for exactly how powerful it is so it might need to be nerfed or removed.