Can I use 18AWG wire instead of 22AWG when doing guitar pedal modifications?
Very few circuits are designed in such a way as to require a minimum resistance or inductance of the interconnecting wire.  Audio folks sometimes do some odd things, but probably not that.  So while we can't technically know without examining the specific usage, electrically this will probably be fine.
The more practical issues you might run into would be related to the larger size and stiffness in getting things in place, and the greater heat perhaps needed when soldering (which might flow to connected things causing collateral damage to insulation, plastics, etc).  In some cases a larger wire might create a mechanical strain leading to future breakage of a solder join or component, but that can be as much about the details of installation as the actual wire size.
Also consider the relative merits of solid and stranded wire - the latter often being preferred.
Ultimately the actual greatest challenges would probably be related to any inexperience in doing such work.  If this audio gear is valuable, exercise extreme care, as it's quite easy to accidentally damage adjacent areas.   Doing one's own work is of course a noble path, but sometimes it makes sense to get some initial practice on more &quot;throwaway&quot; items.
Sometimes you can scrounge suitable project wire out of things being disposed, too.  Though beware that some cables use specialized wire types which aren't useful for general hookup, for example they may have strands twisted with a string, or be thin ribbons wound around one.  But if it has the hand of a nice interconnect wire, and on stripping appears to be just copper conductor, and takes solder without the insulation making too much of an acrid mess, it can be great.