I have tried (in a bash shell): rtl_fm -f 96.3e6 -s 48000 - | aplay -r raw -f dat -c 1
For rtl_fm, -s sets the station bandwidth, you probably want -r to set the output sample rate instead. You probably also want -W for wideband FM mode.
For aplay, -r sets the input sample rate, and "raw" isn't a valid sample rate. You probably mean -t instead. This works for me:
rtl_fm -f 105.3e6 -W -r 48000 - | aplay -t raw -f dat -c 1
Cheers, Adam.