Playing AAC audio from FLV video in Rockbox

I wanted to have the audio from a guided meditation Flash video on my portable device running Rockbox. After downloading the video to an FLV file, I first tried using FLVExtract. This created an AAC file which was playable in Winamp, but Rockbox didn't recognize it. I had to put the file into an MPEG4 container. For that I used FFmpeg from the command line:

ffmpeg -i audio_from_flvextract.aac -absf aac_adtstoasc -acodec copy final_audio_file.m4a

It's also possible to skip the FLVExtract step:

ffmpeg -i video_file.flv -vn -absf aac_adtstoasc -acodec copy audio_file.m4a

For some reason, this M4A file was slightly larger.