Is this how you calculate this all out?
Close but I wouldn't break down the serial into bits, I
don't think it adds anything.
That is quite a few...
Most of them are spent hanging around waiting for the serial buffer to have some space in it.
So in theory I am within my limits of 9600 baud.
Not sure what you mean by "within limits"
Is this going to cope..?
It depends on what the ISR is going to do.
The biggest drag is the serial output, find a way to cut down on that and things will improve. However the arduino is only a 16MHz processor with limited memory so any audio processing you can do will not be very good.