48/96KHz is talking about the sampling frequency, that is how many times per second a data point is recorded.
Generally most CDs are 44.1
Movies are 48
You can set the bitrate to 128/192/256/320kbps independantly of the sample rate (44.1/48/88.2/96/192KHz)
Reducing sampling frequencies suck. Converting between frequencies that aren't multiples of each other also sucks. Sometimes you have to deal with it.
It's advisable not to change the sampling frequency of the source material unless you have to.