Changed sky? You mean a different Ceiling texture, certainly, unless there's more to SRB2 editing than I knew.
BTW, while setting up the game on my school's computers (well, temporarily at any rate; all but the gamedata file was deleted at the end of the meeting), I noticed that if SRB2 can't find any sound systems (ie: no speakers), it'll crash instead of defaulting to a no-sound mode. Oddly, just "-nodigmusic -nomusic" will override this, but still, shouldn't it default to a no-sound mode instead of crashing?