Audio only transcoding

I have two questions regarding audio only transcoding: I use the API to pull an audio file from a video file. As instructed, I don’t pass the videoBitrate so video will not be encoded. The parameters I pass are:

source=file:///.../test.mov groupName=encoder_6 destinations=file:///.../encoded.mp3 audioBitrates=64000 audioFrequencies=48000 audioChannelsCounts=2 keepAlive=0

The JSON I get at the start of the process is:

{"id":1647,"payload":{"arguments":"/.../test.mov na na na 64000 2 48000 na na na na na na /.../encoded.mp3 transcoded_1384787030192520","configId":9,"customData":123,"fullBinaryPath":"/evostreamms-1.6.4.2600-x86_64-MacOSX_10.8/bin/emsTranscoder.sh","groupName":"encoder_6","keepAlive":false,"operationType":6},"timestamp":1384787030,"type":"processStarted"}

and at the end:

{"id":1660,"payload":{"arguments":"/.../test.mov na na na 64000 2 48000 na na na na na na /.../encoded.mp3 transcoded_1384787030192520","configId":9,"customData":123,"fullBinaryPath":"/evostreamms-1.6.4.2600-x86_64-MacOSX_10.8/bin/emsTranscoder.sh","groupName":"encoder_6","keepAlive":false,"operationType":6},"timestamp":1384787036,"type":"processStopped"}

The returning mp3 file however is 0 bytes (actually, just realise now, disregarding the extension, is it possible to create AAC or MP3 files?) Secondly, assuming the above is possible, is it possible on a live stream (multiple streams encoded at the same time) to have an audio only stream among the video streams? And if so, how do you configure that using the API? Thanks!