Music: It seems you just need to wait the end of the first music you can hear in the corridors. You'll need to have at least the Garrison_District_grsnWallRoomClimbingPhys.prp from Until Uru in your "dat" folder and have updated the .sum file (or use the UU installation). Drop yourself into the Wall area thanks to the suit machines (not flymode), and the first music will starts, followed by the second. (else, you can simply find the music file in the "sfx" folder)
About ABM: it depends, in fact. If you are using an Age Beyond Myst install, you must use the "ABM" patcher, and then copy the original "UruSetup.exe" in your Uru folder (but I also heard you could copy UruExplorer.exe instead, but I'm not sure for ABM version). For Complete Chronicles, you must use the "CC" patcher, and then copy the original "sp.dll" and "UruSetup.exe" to your folder.
And flymode disables almost everything, yes. There are still the "visregions" (the regions that hides objects if you go out of them).