NWZ A815 To answer your question the Op Manual says it takes MPEG-4 and AVC format. The Walkman came with a Siperman Trailer that is in an MP4 format

the following file types are for your player
Sony Walkman MPEG-4 Movie (.mp4) or Sony Walkman MPEG-4 AVC Movie (.mp4) You can also use Sony Walkman WMV9 Movie (.wmv) as output format since Sony Walkman NWZ-E436F supports WMV video formats encoded with WMV9 video codec. The video size for Sony Walkman NWZ-E436F is 320 x 240.
If you need further assistance please feel free to follow up with a comment and i will be here to assist you. If this assisted you please feel free to rate the solution. thanks and have a great day. -Midwest-tek

try to download the klite mega codec to become playable on any media player.

here the link

cheers
CHARCOIS

★★★★★
omg i took me about a year to figure it out but i finally got it, i'll tell step by step.

1. you need a video converter because the walkman needs .mp4 files (i use Daniusoft video converter, you can get it via this link)
2. download ur files then plug ur walkman in and then go to start- my computer- walkman- storage media- video
3.go to start-my documents- then try to find ur .mp4 video (it mightby inthe my videos file or if u downloaded a converter like theDaniusoftvideo converter its gonna be in that file)
4. drag and drop the video file to video on ur walkman and ur done!

same thing with pictures except u go to pictures instead of videos and then drag and drop the file

sorry if u dont understand it, its kinda hard to explain :]

Walkman supported video: MPEG-4 and AVC files.
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Tutorial:
How to Convert DVD's and YouTube Videos(Background music) to Sony Walkman?

I think ur MP4 file is in wrong resolution for sony walkman, you could try this Daniusoft video covnerter, It is already optimized for output setting for sony walkman ... easy-to-use.

As for me, i all through use Daniusoft video converter suite to convert various videos even Movie DVDs or HD videos to sony walkman-friendly format, and then put them to my Sony Walkman Video NWZ-S616F MP3 Player for watching ...

Video Capabilities
Video Format MPEG-4 • H.264
Max Video Resolution 320 x 240
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Daniusoft video converter: Convert all mainstream video AVI, MPEG, DivX, WMV, MP4, MOV, FLV, MKV etc. to Sony walkman video fast and perfectly.

Walkman Video Converter Convert video espeically high definition video to Sony Walkman/ Sony Ericsson Walkman phone MPEG-4/AVC, MP3, WMA and M4A. Edit video with unique editing features to trim, crop, adjust effect, add watermark, etc.

This Walkman Video Converter can also extract and convert audio from video to Walkman audio formats including MP3, WMA and M4A. This Walkman Converter completely supports Sony Ericsson Walkman phone, Walkman Bean, Walkman Buzz, Walkman Fitness and more.

it only suports MPEG-4 AVC format im looking for a converter as well currently because im having the same problem

MP4 format doesnt work convert it to AVC or MPEG4 those work