The 'NBFM receive' block's 'audio rate' must be stated as it is the value for the output sample rate of the block. Yes, this can be the same as 'quadrature rate', which is the value for the input sample rate of the block, or what sample rate it expects to receive, however the 'audio rate' must be the result of dividing 'quadrature rate' by an integer (also known as audio decimation in the next paragraph), in the latter case this would be '96000/1=96000.
Note that when using the 'NBFM receive' block for example, if the 'quadrature rate' was '96000' and the 'audio rate' was '48000' then the 'AM demod' block would have the equivalent values of 'channel rate' = '96000' and 'audio decimation' of 2.