Correct audio settings are 48khz for all digital video. 44.1khz is for CD audio.
All audio should be converted to either wav or aif 48khz before it is imported into FCP. FCP will not play mp3s properly, even after a render.
Regards the slo mos, your problem is more likely that you shot at 24p not 60i. You get smoother slo mos when you shoot interlaced at a higher frame rate, not low rate progressive. Also you can only judge that sort of thing properly on an external interlaced monitor.
btw, you can find heaps of answers to these oft answered questions by searching this forum.