If the brown areas started with the mower wheel tracks and then spread,
it sounds like you may have spread a fungus/disease with the mower.
I'd get the company that installed it out pronto. Also, if you have an
agricultural service available, usually through the county, I'd take a
sample to them for an evaluation. If the sod company isn't sure what
it is, I'd ask them if it's OK to put down a fungicide. This won't
harm anything and if done quickly could save the lawn. I'd also talk
to the sod company about watering levels. Keeping grass constantly wet,
plus excess nitrogen and hot weather are a presecription for problems.
I'd also start taking pics at intervals, which will be valuable if you
get into a dispute with the sod company.