Regarding slew rates, what would be the minimum for amplifying audio using an op-amp?
the slew rate needs to be \$\geq 2\pi f V_{\text{peak}}\$ to not impart distortion on a sinusoidal signal.  

\$f\$ = frequency of the sinusoid and \$V_{\text{peak}}\$ = peak voltage.

So in your case you'd probably use 20kHz for the frequency and whatever your \$V_{\text{peak}}\$ is to find your minimum slew rate.