attenuate record level when rate approaches zero

[catfact]

while recording, as rate nears 0 we are essentially downsampling the recorded signal to SR=0, and causing all frequencies to alias. so sweeping rate smoothly between positive and negative always causes noise. we should simply apply an envelope to attenuate the signal in this case.

[catfact]

added sinusoidal fade around rate==0 in commit id 107dcad27604c8b0300d40cf9c2c5f05ce0525e9
could probably use some refinement, e.g. widening "deadzone" (and certainly could use refactoring)