I went through troubles to get work scrobbling for mp3 files synced from pc to my 3G iphone through the gtkpod. I finally found out that the tracks scrobbling works only with audio rate 22050. Scrobbling stops working when the song is sampled 44100 times per second.
ffmpeg -i source_video.flv -ar 22050 -ab 128 -ss 00:00:01 -t 00:00:10 sound.mp3
ffmpeg -i source_video.flv -ar 44100 -ab 128 -ss 00:00:01 -t 00:00:10 sound.mp3