After trying the online tool referenced, I found two major (show-stopper) issues :
- limited use (you need to register/pay for unlimited conversions/batch conversion)
- unusable AAC files (bad bitrate).
After looking a little, I found an OPENSOURCE and FREE solution working at least on Windows and Linux (I don’t have a Mac computer) : Fre:ac (previously BonkEnc).
In the filter options, you need to activate two filters : the channel (set to mono)- and the samplerate (set to 24000).
In the encoder, you need to put FDK-AAC and configure it for 64kbps and AAC (AAC is on the second tab).
Il allows batch-encoding (but better go to optios->encoder and set file name to something without a directory name for example - ).