Hi suchb,
There is an alternate method to recofigure i2s1 (i.e settings its pinmux) mentioned at link. Suggest you to try this as this does not alter your customised DTB that you have flashed.
Above method helps to edit pinmux function in the pinmux table for i2s and generates DT which is further converted to cfg file that needs to be reflashed for the pinmux to take effect.
Thanks