ffmpeg spits out the following warning when the audio rate of the source is
too low for the requested bit rate for libfaac to code:

[libfaac @ 0x82487f0]libfaac doesn't support this output format!
Error while opening codec for output stream #0.0 - maybe incorrect
parameters such as bit_rate, rate, width or height

This is solved when the preset contains a -ar option. I found by trial and
error that the following are LOWER limits:
@192kb: 32000 Hz
@128kb: 21334 Hz
@ 96kb: 16000 Hz
@ 48kb:  8000 Hz

For reasonable results (by looking at existing presets) I propose:
@192kb: 48000 Hz
@160kb: 44100 Hz
@128kb: 32000 Hz
@ 96kb: 24000 Hz