NW-S644K

The NW-S644K added speaker support to the seventh-generation S Series entry with 32 GB flash capability. It brought x-app features and PS3/TV integration to the lineup. With 8 GB storage supporting MP3, WMA, AAC, and WAV playback plus video and photo, it uses a 240x320 TFT display, 42-hour battery, WM-PORT, Clear Audio, DSEE, Lyric Pita, x-app, and built-in speaker.

The K version pushes that same speaker-oriented S identity into a more accessory-driven retail package. This is exactly the kind of variant that appears once Sony is no longer trying to explain what the line is and is instead comfortable stretching it sideways through bundles and use-case packaging. The platform underneath was already stable enough to support that kind of extension.