I'm looking for a spell to go through walls too, ideally lvl 15
I'd be happy to take the lvl 15 version here, but I can't see how "change stone so that it is slightly unnatural" turns it into a passageway, because of the Muto definition is "grant or remove properties something cannot naturally have" (core p78)
Being in the form of a Passageway is a shape that stone naturally can have, so wouldn't that be Rego instead?
Another Muto option would be to make it act like a curtain (like in the Net Grimoire spell) or something else highly unnatural, but that's:
Base 4, Stone+1, Touch+1, Conc+1, Part+1: MuTe 20
With Rego, I worry that it might falls into the Realm of Rego crafting (Covenants p49) and its crazy high Finesse rolls.
By my calculation, it would require Finesse roll of ~12: "simple" task for a simple archway (6+3) that takes a month's work (+3)
It's also permanent, which is not what I'm looking for. I don't see a temporary Rego option for that kind of effect.
Another option would be Perdo to remove stone + Rego to keep it in place, because the target Part of the wall is effectively destroyed, and it's only the area around it that I'm making a better example of itself.
That would be permanent too, except that I have picked the Harmless Magic flaw:
Edit: A slightly unnatural state (base 3) that would work might be some kind of gelatine state that allows you to slowly walk through (idea stolen here)
I'll make a version of Rock of Viscid Clay (Conc, Stone) that gives this kind of property instead. I might even find other uses for it 
I've also seen mentioned PeTe to remove the substantiality of stone, but that's base 5 (HoH:S p37)
Other References:
There's a Minor Lab Flaw in Covenants p119 called Impregnable that involves your lab having no easy access, for example by not having a door.
The suggested method of getting in is MuTe spells
An example MuTe(Au) 25 is in MoH p93).