Don't depend on the FS conference rate being 16kHz

One person has noted that recordings didn't work when the FreeSWITCH
conference bridge was switched to a different sample rate (e.g. 8kHz).

Fix the mixing samplerate on the sox command used for audio import to
run at the audio file rate.

Note that no matter what input rate is used, the output sample rate will
always be 16kHz.