Any Studio integration version later than 1.05.06 will have the 64bit iOS libraries already in it.
Unfortunately the new IL2CPP tool that Unity has used to support 64bit iOS has bugs that stops projects using the FMOD Studio integration from building in XCode.
To work around this, you must search and replace all instances of the string “_3D_ATTRIBUTES” with the string “ATTRIBUTES_3D” in all .cs files underneath the directory Assets\Plugins\FMOD\
EDIT: fmod_dsp.cs line 37 needs to be changed from CHANNELMASK[] to uint[]