In any case, the NWZ-S615F (lets call it the Spod for sake of my fingers), uses the defacto portable media player screen resolution of 320x240. This is good news for you because there are a plethora of free or nearly free (read:cheap) video converters and rippers out there for your Spod. Just make sure the video codec is Mpeg4 - AVC (H.264/AVC) and the screen resolution is set to 320x240 and you should be all set. The other good news is that the Spod can play a wide range of audio codecs, so you shouldn't run into too many issues there.
I've only dabbled in this type of software when I got my last smartphone, so I can't really comment on which free software is best (couldn't stand watching things on such a tiny screen), but you could try Iriverter or SuperC. SuperC had a superior options list, but seemed a bit bloated for what it does and was a huge system resource hog.