This is a tip that probably applies to many Sony Ericsson smartphones, including my K608i. AMR, or Adaptive Multi Rate, makes really small sound files, mainly for voice. However if you create AMR files, the phone's Media player doesn't read them. (The File manager does play them.) The key is to make a 3GP file with only an AMR audio track, this will play, and enables seeking, minimizing, pausing, playlists and all the cool features of the Media player. Apple's QuickTime or possibly the open source ffmpeg programs can create such files.